I don't really see there being a "cap" to healing. My guild leader and main healer runs around with about +2250 fully buffed iirc (and around 240mp5 unbuffed). He sticks with Renew, PoM, and full-rank Gheal (I don't think he downranks Gheal at all), and regularly tops healing charts. He also, of course, tops it when there's lots of AoE damage, as he goes into CoH mode (which, btw, is about the most overpowered heal there is).