You should consider the time of the triggers. Regen, as example, triggers twice for every poison trigger. Unless I miscalculated it, poison and regen triggers null themselves in damage for this actual configuration.
Characters can have the maximum of 9999 hp. However, it is unlike to happen without grinding and maximum HP boosts by level. Enemies, however, can have nearly 65000 hp (a little more, I don't remember exactly the number). Because the damage/healing is based in maximum hp/mp, the healing/damage for monsters is always higher when compared with characters.
I put the cap of 999 damage/healing to try to balance the damage between characters and monsters. The cap can be easily removed. However, it is likely regen/poison for monsters will do 9999 damage/healing by trigger for enemies with higher HP. Is it acceptable?
It is common to have a character reserved for healing. A Cure 3 spell can easily heal almost all HP instantly. Regen and poison will take many turns and it is unlikely they will trigger more than once by turn, with few exceptions. Regen can't compete against Cure 3 in usefulness if the HP healing of regen is low.
I don't know where the time of the triggers is set and I can't change them. However, I can easily change the amount of damage/healing, as long as it is X/16 of maximum HP or MP. Seizure and phantasm will share the same amount, however, because of technical restrictions.
I am open to suggestions for a eventual patch update, if the arguments are valid.
NOTE: I like the fact that stamina isn't used anymore because it raises poison damage in the original game. If you raise stamina with espers, it won't hurt you now. :-)