Currently FP regenration is based on Constitution only. Also, Combat XP is dependant mainly on FP regen. All other things equal, FP makes a huge huge difference. Magic XP isn't very dependant on regen at all, really, so not comparable.

What this means is, if you are interested in doing combat, not maxing your constitution is stupid and suicidal. It will put you at a heavy disadvantage. Making one stat so very powerful I don't think is a good idea. Not to mention Endurance would seem more natural for deciding FP regen.

So, instead of having FP regen = Con/3 (affected by hunger/thirst), why not:
FP regen = (Con + end + wil)/9 (affected by hunger/thirst). It should be base, I think, or spellups might affect it too much. I can't recall what spells affect those stats, if any.

Having it based on three stats instead of one would take away the need to max that one stat to be competitive. No other stat is that essential, I think.

Also, having to be Liquid Blob!/Glutton to have max regen (for Mana/FPs/Hps) seems weird. As long as you are at least satisfied, why would regen be lower? If anything, being Liquid blob!/Glutton should lower regen, since you are so bloated and such.. Then again, that'd require us to be able to chose how much we'd eat. Gorging yourself would be bad.