I forgot to post the evidence of this, but I figured I would do so considering the number of people questioning this off-site.
Ansem was not amped by Kingdom Hearts in his fight. The
description of Kingdom Hearts III's character entries flat out says otherwise. Even if you look at the context of the fight itself, Ansem only gets to Kingdom Hearts
after the World of Chaos boss fight
to ask for "supreme darkness". That's the only time Kingdom Hearts plays a role in that fight, and
then it completely one-shots him after.
Then there are some arguments about Xemnas still as well. Not only does
the description of his character entry note it was completely artificial, but the actual thing is
completely shattered by Ansem the Wise before Xemnas merges with it. Xemnas even
says this Kingdom Hearts was ruined, so we can't scale it off the other artificial Kingdom Hearts. It serves as an unquantifiable boost for Xemnas in this case.
Also, you heard that right,
both of those instances are artificial,
according to Ultimania. So no, we can't scale it off the OG Kingdom Hearts. There is absolutely nothing that indicates that.
There are some people trying to scale Ventus-Vanitas off of True Kingdom Hearts too because of the X-Blade. A few problems here... Firstly, the X-Blade itself
does not scale to Kingdom Hearts. Just because Master Xehanort called it the counterpart doesn't mean the AP is the same, it's in reference to how its
correspondent function is to help
summon or
seal away Kingdom Hearts. It can help control its power, but it doesn't scale to it physically is the main point. It wouldn't even scale regardless because
the X-Blade was incomplete because of the inner clash between Ventus and Vanitas,
with the glossary even saying it was incomplete too. This is a very similar problem to the Xemnas situation.
And now we get to the Xehanort situation, which is probably the only genuine contestable evidence for Tier 3/2 Kingdom Hearts. The whole thing about the X-Blade not scaling applies here too. However, Xehanort does do some impressive feats like corrupting Kingdom Hearts and calling a beam of light down from it. The problem comes with the fact that Xehanort was still not using its full power, though.
Melody of Memory explicitly deconfirms this when the narration mentions they were going to
stop its power from being unleashed. It mentions how Xehanort completely abandons his plan before he can use True Kingdom Hearts' full power too. In the vanilla game as well, Xehanort still implies that
all of them would have been purged if it hadn't been sealed away again. So that means the prior feats weren't utilizing the full power of Kingdom Hearts in those instances and/or were impressive enough to be on that level. Scaling Sora to something that's going to outright kill him with ease along with everything else makes absolutely no sense.
It is for these reasons that any sort of scaling to Kingdom Hearts' feats are incompatible with the characters. To do so would be literally ignoring the context of each and every situation.