Hello, yes. I will throw in my two cents.
Take note I don't read a lot of comics but I have seen the pages the feat takes place in and know the context.
That being said, I think discounting the feat as inconsistent is dumb. Are comics inconsistent? Yeah, most of the time. But let's compare an actually inconsistent feat with this.
Batman punching Spectre is inconsistent because he only does it once and never again. He also has a dozen more feats of fighting street level villains than anyone even close to Spectre's level of power.
Now for Supes' feat. He did it after sundipping a considerable number of times. He's clearly amped up by the energy he absorbed and makes it clear in his dialogue.
See the difference between both feats is that Supes actually gets stronger and that is made abundantly clear to us, the audience. As such, by our very own standards, a tier jump is not out of the ordinary.
As for the other arguments that dipping in a few suns for at most a couple of minutes isn't enough energy to reach 1-A, that's right. However, the feat of punching World Forger should take precedence over attempting to calculate the proportional strength gained from spending time in the sun. Why? Because the former feat only happened once and is by definition impossible to be inconsistent because there's literally no other feat to compare consistency to. On the other hand, Supes has had many sun dips in the past, and I sincerely doubt he has consistently shown a proportionate boost in power to how long he has spent in a sun.
But then again, this dipping event was specifically stated to be special since the scan blatantly states he moved faster than he ever did before and at speeds beyond physics. I'm honestly surprised there is a debate here when it's spelt out so obviously for us.
In conclusion, Supes with multiple sun dips should be 1-A. It is inconsistent to absolutely nothing.