I don't know if anyone cares about Bio III anymore, but SE made it not suck:
https://www.bg-wiki.com/bg/Bio_III

I would bet it caps at 20 damage/tick, if anyone has the gear required to try (or if it even exists).

Other things that probably cap at 500 Enhancing skill:
* Phalanx II (5/5) uses the same formula as Phalanx above 300 skill and I can't hit the cap with 489 skill. (-35 damage cap probably)
* Enspells have the same formula they always had. They probably cap at 500 skill (30 damage).
* Enspell IIs are as worthless as ever, and also unchanged. They probably cap at 500 skill (30~60 damage).