Okay, there's a lot of dodgy mathematics floating about here (I study mathematics btw) which really isn't necessary.

Serapy loves pointing out the obvious point that nothing is disproven 100%. This is a literary debate, not mathematics so you're not going to get anything 100% disproven. The FAQ says what's true, namely that it's "highly unlikely". In fact, I would say it's about as proven wrong as it can possibly be in the context of literary interpretation in the sense that if you accept R=U as a valid interpretation you might as well accept Irvine=Ultimecia or Cactuar=Ultimecia as valid interpretations. Why?

Firstly, there's no good evidence. Vost and Serapy both like mentioning "evidence" without ever specifying it. In fact that's because all 'hints' found to support R=U have been debunked, or shown to not actually imply R=U (in the sense that it's much more likely that they imply something quite different). Serapy or Vost cannot name any good evidence because it doesn't exist. There are at most one or two hints left which haven't been blown out of the water, and those do not make up enough evidence for such a strong statement as R=U.

Secondly, the game not only offers no evidence, it throws every conceivable obstacle in the way of making R=U even possible to begin with. No extended sorceress lifespan, no mention of Adel's seal preventing aging, Ultimecia actively tries to kill herself and Squall and the game AND the Ultimania all imply a quite different background for Ultimecia. That alternate background implied is not, as Serapy claims, flawed. There's no flaws whatsoever in it and Serapy if you think otherwise I challenge you to name even one. The details can of course be found in the Time/Ultimecia Plot FAQ at GameFAQs.

So we have "R=U", a theory which requires you to literally conjure a scenario from out of nowhere to even make it possible, and even if you do so there aren't any good hints at all implying the theory to be true anyway. In addition we're given a perfectly plausible background for Ultimecia which is even supported by the Ultimania AND Dissidia.

Saying "nothing's been 100% disproven" anyway is all well and good, but I maintain that there's no meaningful difference between R=U and Irvine=U. As such the theory is most definitely "extremely unlikely".