What exactly is different about Kefka than Kato or Sephiroth? At the very least, the Fanatics' Tower is very very possibly optional, given that MagiMaster kills you + sets you back nearly an hour the first time you fight him unless you are prescient/lucky/FAQing, not to mention the risk of being wiped by L.90 Magics. Quick if FAQ-bait extraordinaire, but at least you had the good grace not to hype that. <.< Level 40 is also quite high if you skip some sidequests, though weirdly FF6 endgame level has more to do with "do you bring Celes to the Floating Continent" and "do you get lost while you only have a few PCs in the early WoR" and other midgame levelling decisions than anything else. Still, the impact on magic damage of a few levels isn't really too relevant.
-Kefka has 62k HP, so an average of, say, 5k (which I find reasonable enough, offhand) kills in 13 hits = notably above PCHP (1.42x to me, should be 1.24x even to you). And Melfice does notably below PCHP damage (low 2HKO if you learn the HP boosting skills, and given your past discussion of Sephiroth, you really should, given how much easier an HP boost they are! but even if you quibble this, that's still not a OHKO, the real debate here should be over 2HKO vs. not).
-It's rather hard to reliably have the PCs you want for Kefka, especially with the last form IDing 1-2 people in immunity-ignoring fashion as a death counter, so specific PC strategies (Bum Rush, Gem Box, and Morph all qualify) are going to have a reduced effect on the averages somewhat.
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Also (and this isn't aimed at anyone in particular), what the heck does Yuri2 do to beat Ryu4? Ryu4's offensive buffs are much more potent than Yuri2's defensive buffs, especially since Shadowalk is half-ITD anyway meaning the defensive buffs don't even work very well. And Shadowwalk -starts- at OHKO damage. Stat topic doesn't actually list max-might Shadowwalk but it should be easily into 5 digits. Rika could win this only because she could do the evade buff thing, Yuri doesn't have that.