Potential unlocked Gotenks

When Old Kai finished unlocking Gohan's power he told him to try transforming into a Super Saiyan but instead of Gohan going Super Saiyan he went to his Mystic form. From there Goku and Supreme Kai were stunned that Gohan had so much power. Goku even commented how Gohan could be that strong without being Super Saiyan. Old Kai made some comment on how Super Saiyan was too flashy or inconvenient depending on which translation you read or heard and he made it so Gohan wouldn't need to be bothered with it and how transforming into a Super Saiyan wouldn't make a difference anymore. He pretty much implied Gohan couldn't get stronger anymore by transforming. Since transforming wouldn't give Gohan anymore power it is assumed Old Kai magically adjusted Gohan's body to reaching all the power Super Saiyan transformations would give him without transforming, thus bypassing the strain the transformations put on his body to make him more efficient. Basically Gohan reaching SS3 level of power without needing to use the form and having to deal with the strain Goku and Gotenks had to deal with. That was how it was the Z anime and original manga framed it. This seemed to hold true since Gohan didn't transform to fight Majin Buu when he got his ass handed to him after Buu absorbed Gotenks and Piccolo.

Now I think it is up in the air. Toriyama has retcon and made changes to canon since he revived the franchise.

Okay, let's get some things straight about Old Kai's upgrade.

Things he specifically said:
1) It's accessed like going Super Saiyan, but isn't as tiring.
2) It is a transformation: Gohan's eyes/hair look more like they do when he's a Super Saiyan.
3) When the transformation is done, Gohan is stronger than he was normally.

Things he didn't specify yes or no:
1) Gohan will never be able to go Super Saiyan anymore. 
2) Gohan will ALWAYS be at Mystic level; even while asleep, he's stronger than Super Buu.
3) The upgrade is permanent.
4) What exactly "beyond your normal limits" means.

Just because Old Kai didn't specify something, doesn't mean it's wrong to interpret the Mystic upgrade in certain ways. There are a lot of different interpretations:

DBGT's interpretation: the upgrade is not permanent, and by the time of GT it is gone. Completely. Gohan is now only able to go Super Saiyan. (This happened, I think, mainly as a combination of GT's writers being stupid and forgetting, as well as Goku wanking rendering all other characters moot.)

Salagir's interpretation: Gohan cannot go Super Saiyan anymore. The upgrade is permanent. Gohan is as strong as he could ever be, stronger than he'd be if he'd trained perfectly his entire life, and cannot become an ounce stronger. Plus absolutely no fatigue. When Gohan is 99 years old, he'll still be at his Mystic level of power.

Super's interpretation: The Mystic upgrade is a higher transformation. Gohan can still turn Super Saiyan, though Super Saiyan is nothing compared to Mystic. He loses his Mystic power if he doesn't train. Gohan can still apparently get stronger (?). To be honest, likely the only reason Gohan can still turn Super Saiyan is because Toriyama forgot about Mystic and had to be reminded.

Ashanark's interpretation: Old Kai's magic has two benefits, namely 1) allowing someone to fight at max without getting tired, and 2) releasing any "hidden" power the user might have. If Goku and Vegeta got the Old Kai upgrade, they wouldn't be that much stronger, but they'd have better endurance. Gohan has a huge reserve of power that he's learning to control throughout his life; as a child, it only came out when he was angry; as a teen, it allowed him to access Super Saiyan 2. When Old Kai unlocks Gohan's power, he is only unlocking all of Gohan's hidden strength; he is not making Gohan as strong as he could ever possibly be. (That just seems un-Dragon Ball-like to me.) Gohan can still train and get stronger, it's just there is no more "hidden power" to unlock or draw upon when he's angry; from this point in his life, all his power must come the hard way. Had Gohan already achieved SS3 before getting the upgrade, he would've been even stronger.

With all this said...
To me, it doesn't seem likely that Old Kai's magic was merely an upgrade to Gohan's base, and that he could still go Super Saiyan and become even stronger. Why did he not transform against Buu, then? The only options are that he is stupid, or transforming into a Super Saiyan or SS2 wouldn't help him, or he cannot transform anymore at all.

At the beginning of the Buu saga, Gohan's "potential," the absolute strongest he could become, wasn't his fully-powered base; it his SS2. Similarly, Goku's full "potential" wasn't his fully powered-up base form, but his SS3. If Gohan isn't fighting at SS2, he's either holding back or else not reaching his full potential. If Goku's not fighting at SS3, he's either holding back or not reaching his full potential. So when Gohan's "potential" is unlocked, I doubt Old Kai was saying "I'm just making your base form stronger, you can still transform if you want." That's making quite a lot of assumptions that aren't very obvious or natural. Old Kai basically said Gohan wouldn't need to transform; therefore, it's more logical that Mystic Gohan is fighting at whatever his hypothetical SS3 level would be; not that his base is made SS3 level, and he can still transform on top of that. (Though I admit, that'd be freaking awesome.)
 
@topic
I imagine Goten and Trunks, at the same level of training, are loads stronger than their dads. This is why they can turn Super Saiyan at freaking 7 years old. They have more potential than their dads. So, going by the generally accepted interpretation of how the upgrade works, I think:
Mystic Gohan > Mystic Goten/Trunks > Mystic Goku/Vegeta
Which means a Mystic Gotenks, having stronger components, would be a good deal stronger than a SS3 Vegito, or even a Mystic Vegito.

Where are you getting that Gohan couldn't? There is no evidence that only full-blooded Saiyans can achieve SS3. Goku and Gotenks achieved it; therefore, the natural assumption is that Gohan can, too. While I personally like to think that SS3 is an unnatural transformation that only dead people and fusions can achieve, I readily admit that's a pure headcanon that seems to make sense. Similarly, while there's nothing to contradict the statement "Gohan didn't turn SS3, and he is a non-fusion hybrid; therefore, non-fusion hybrids can't go SS3," it's not the best logic.

I'd like to get a couple of things straight myself:
-I am not debating what has been established as canon regarding the Mystic form. I've said this repeatedly, I'm well aware of where the anime/manga stands on the Mystic transformation. Originally I wasn't aware that it was already made clear in the anime (albeit with some errors on Toriyama's part as was mentioned earlier) that Gohan was magically altered so he no longer needs to transform to access his SSJ powers. I'm pointing out that it's referred to as a potential unlock, which by definition should have put him at a level he would have been at had he trained hard enough to reach his full potential. Since it's a magical powerup, the term is inaccurate, and that confuses people. That's really all I'm pointing out, they should just call it what it actually is instead of making it seem like they intended it to be one thing then retconned it to be something else.
-I didn't say at any point at all that the potential unlock wouldn't give Gohan access to a transformation if he could achieve it through training. If he's capable of transforming and that transformation's power was magically added through the potential unlock, good for him. But I'm pointing out that it hasn't been established that hybrid Saiyans can go SSJ3. Honestly if anything DBZ has implied that transformations work a lot differently with Saiyan hybrids. What Goku himself comments on is essentially meaningless, he's just speculating and has no clue if Gohan is physically capable of going SSJ3. Has any hybrid Saiyan transformed the same way Goku and Vegeta did? Gohan turned SSJ2 less than a year after he unlocked his first form, Goten and Trunks achieved SSJ without even trying when it took life-rending horrors to make Goku and Vegeta transform, and Future Trunks gained a form that eclipsed SSJ3 in much less time than it took Goku to gain the form. My point being we don't know what kinds of forms Gohan's power boost could have granted him if they weren't magically integrated into his base form. Hell he could go Super Saiyan Purple before he goes SSJ3. With a full-blooded Saiyan, we know the form can be accessed, but with a hybrid we have no clue nor evidence that they'll have the same exact transformations.
-I'm not trying to over-analyze anything. If I was I probably wouldn't have been so quick to point out that bad writing is the main explanation here. People who over-analyze are the guys who try to explain something using canon facts and aren't willing to accept that bad writing is the cause. I would be amazed of any anime actually was 100% accurate, DBZ is probably like 20% accurate canon wise at the very most. My point has been and will remain that the lack of clarity and logic here simply irritates me. I don't expect perfection but we've got a series writer who's forgetting pretty well known stuff about his own series and making up nonsense crap to fill in the plotholes. That's not good for any series and it's crappy writing. DBZ has never been good at continuity, they get a lot of stuff wrong. What annoys me is when they use a pathetic explanation instead coming up with a more creative and sensible reason for something. 

I have to say though, for people who are making a case that you shouldn't have to be 100% accurate about a series, you sure have a few guys on here who love to poke fun when you get some of the details of something wrong. I don't mind being trolled so let me have it, but you all sure make a big deal out of it if someone doesn't know every little detail of a series. I like DBZ but I'm not a nut about it, if I'm going to have people kick my ass over not knowing everything there is to know about it that would really take the fun out of being here. I remember a lot about the series but the last time I watched the full Buu Saga was ten years ago, my memory isn't going to be perfect. As you and I have both pointed out the series is pretty well known for their continuity screw ups, I'd be very surprised if I was the only person on here to get something like this wrong. I'm mainly referring to the first few idiots that replied to this thread by the way, not you.