Off the top of my head, I know that AB ignores multiple casts, meaning it doesn't stack if casted by the same user, but allows multiple stacks from others. The recharge portion of the buff isn't enhanceable or affected by other powers and the movement resistance isn't either. The regen and recovery aren't constrained by those conditions as far as I know though. I'm just not sure how by much it can be boosted. I'm speaking specifically about the Defender version of Empathy.
Edit: Oh, but I meant to add, that I didn't know Power Boost doesn't affect regen/recov values. Has that always been the case?