I am not saying that the Japanese "Cefca" spelling is incorrect in any way, what I am saying though is that I don't see any good reason to use that spelling in English versions of the game when the pronunciation "Kefka" has already been well established.
Every FF6 enthusiast knows that his name is normally pronounced "Kefka", and almost nobody says "Sefka".
This seems like a classical "if it's not broke, don't fix it" kind of scenario.
There is no concrete reason to change "Kefka" into anything else, just like there would be no reason to change "Vivi" in FF9 into "Bibi", or "Auron" in FF10 into "Aaron".
Another completely pointless change from FF Tactics is the change from the wonderfully simple "Magic Song" into "Magickal Refrain" - like, what?

Compare these changes to changing "Nuke" into "Flare" in FF1, or the horrendous "Fire Bracelet" into "Fire Breath" in FF Tactics, and I think you can see what I mean.