I think everyone has theories about their favorite games, it's proving it that's the challenge. Any good theory in science is falsifiable, that is, can be proven false. I hold game theories to similar standards. If someone proposes a theory (myself included) is their job to prove their idea right, not for someone else to prove them wrong.

Palette swapping goes all back to beginning of the series. The question is was it to save on work or was there a deeper reason. A few years ago I actually suggested (tongue in cheek) a B=T theory, since it is technically possible that Bahamut could have been corrupted in the future by Ultimecia into Tiamat. Both do use similar attacks (Mega Flare vs Dark Flare). But in the end I doubt it simply because Tiamat has been protrayed in most previous FFs as a different being from Bahamut (and usually with more than one head).