Hammilton-
I don't think coryantheidine plays much of a role in the "ceiling" effect of PL. I'd wager mitragynine is the culprit for the "wobbles" (nystagmus), dizziness, etc, due to it's adrenergic action. I've tried isolated mitragynine, and it does produce these effects, and is much less pleasant than plain leaf. Another oddity....large doses of UEI or FST allow me go take much larger doses of plain leaf concurrently without the side effects. I'm gonna guess the straight opioidergic effects of these extracts can negate some of the nasty adrenergic effects of mitragynine...
It varies incredibly what people's "ceiling" dose is. I used to be able to take 15g to good effect. Then my max became 7g....anything over and I'd get the "wobbles". ...hence the 7x7 dosing regime I was on for a few years before I finally quit. Hard stuff to kick at those levels....even with the help of Ibogaine. Ibogaine worked great on the straight opiodergic part, but I was still left with the ever lingering SNRI and calcium channel blocker like withdrawal.....those are the best analogies I can come up with, whether or not they are pharmacologically correct.