Hi Chris,
We currently have a '02 or '03 customer running a setup very close to yours that is making 517 RWHP with one of our Magnuson M112 units.
He does have a built engine, like yours, and is using aftermarket Comp cams with a Cobra dual pump setup and 60# injectors. I believe he has a larger crank pulley and approx. a 2.7 or 2.8" upper pulley making 16# of boost. We believe he has a bit more power left in this setup as he is not using our heat exchanger or fluid pump because he has a Saleen that originally came with a M90 kit. He is still using the original Saleen items that I just mentioned. We recently sent him our pump/h.e. and we expect he will pick up another 15-20 HP. In our estimation, that is likely the limit of the Magnuson M112, but this is surely much higher than most people would estimate it to be.