Well, first of all greater consumptive field (Libris Mortis) affected by Persistent Spell (SRD: Divine) is a 13th-level spell, requiring four Improved Spell Capacity feats. So given that we're already talking an epic caster, we can probably accept a little grandiosity. However, I'm not sure it's all that bad. [EDIT: Presumably the caster is neither Good nor dedicated to a Good deity.]
Creatures must have less than 9 hp and be within 30 ft. of the caster to be affected at all. Creatures become immune to that day's casting of the spell if they save against it once.
The Strength bonus shouldn't stack; it's subject to the "same source" rule. [EDIT: If you're using the Spell Compendium version instead, the total Strength bonus should be +2 × (creatures killed); check each time a batch of creatures is killed to see if the bonus is replaced.]
For the same reason, the temporary hit points shouldn't stack; only the greatest number siphoned should count. The wording is weird though. It looks to me as though you're supposed to get [1d8×(creatures killed) temp hp]; the alternative, [1d8 temp hp] (roll each time a creature dies), seems contrary to the intent.
The caster level improvement stacks, but it's limited to half your caster level. There's no change in the effectiveness of greater consumptive field based on caster level aside from its duration, which has been replaced by the Persistent Spell. [EDIT: I see there's some kind of cheese surrounding casting the thing again using your newly improved caster level, but I wouldn't change the ultimate cap in that situation.]
HTH. HAND.