Okay, after reading through the comic, I agree with most of the others. Mxy's amp
would only be a comprehension amp stopping him from going mad, and as others have pointed out, he didn't bring him to the 6th dimension,
but rather a Galaxy with no sunlight. I also agree the feat doesn't happen within the 6th dimension, that's pretty much just the title of the comic. The only other evidence I could find is too vague. While Superman is said to be in a place "
beyond everything", that could just mean outside of everything else, as a reference to the Galaxy he's in. The comic also
mentions the 6th dimension while Superman is in the Galaxy, but this is just continuing how
previous issues explained dimensions in the first few pages. Considering
Alpheus would replace the current Multiverse with his, the feat should stay at 2-A, unless the Multiverse is downgraded to 2-C.
Also, real quick I'd like to touch up on Superman possibly scaling to Alpheus. While I get the whole "everyone can fight everyone in comics" thing,
Alpheus himself thinks a sundipped Superman could kill him, and for that reason, he wants to move the suns away. So, it's not just Superman coming down and knocking him down, but something that Alpheus believed would even kill him. As well as this, considering in-Multiverse Alpheus is 2-A, it wouldn't be an outlier for this Superman to do so, since destroying the Multiverse should put him on a similar level. Although, I think the Anti-Monitor thread might downgrade Alpheus as well (correct me if I'm wrong), so this may just be a moot point.
TL;DR: In my opinion, Superman didn't destroy a 6th dimensional Multiverse, just one that's equal to one currently rated as 2-A, so the feat should stay at 2-A unless the Multiverse is upgraded or downgraded. Superman also isn't amped by anything aside from the suns, and can possibly scale off of in-Multiverse World Forger.
Edit: Whoops, I forgot to add this little bit supporting why it didn't take place in the 6th dimension. Alpheus brought Batman, Flash, Wonder Woman etc.
into the 6th dimension with
no problem. So basically, Mxy telling Superman only he could handle the 6th dimension due to Mxy's comprehension amp was just him getting the JL to believe only Superman could go there, and even Batman didn't need an amp to survive in the "6th dimension".