Well the injector scaling shouldnt change if you're using stock injectors. The fuel pressure difference should matter. If there even is one. I have that regulator on my car too. Reads like 44-45psi.

Sounds dumb but did you turn up the idle with the BISS?

I would try to get it running first with the stock mafs before adding the GM as well. In fact just sell it and get an IAT sensor and a wideband o2. Speed density ftw.