My ignition advance multiplier (IAM) was at 0, which turns off boost control, forcing wastegate spring pressure.

It sounds like you might have a boost leak that's preventing the wastegate from closing. Its a bleed type system, so if you're leaking, it can't get boost to the actuator to force the wastegate closed. Or is it the other way on those turbos... Regardless, you should make sure that none of those little hoses have a split or crack in them.