I've done some searching and i can't come up with much.

What is the aprox whp that can be achieved using the stock fuel system + walbro pump & injectors?


At what point is a FPR needed, or ever?

 

You probably won't need any sort of FPR unless you need to increase the rail pressure, you can sometimes do that to get a bit more flow out of injectors that are a little smaller than your car demands. But if you size the injectors to the application then its unecessary..

Fuel Pump and Injector upgrade are really the only mods needed for levels to 575whp or so (give or take, it depends on your engine management, tune, and ofther factors) At that point you may have to run a piggyback pump (Secondary inline pump) to maintain the pressure and volume, or possibly upgrade the entire fuel system to larger lines (You rarely hear about people doing that unless their going larger than a GT35r anyway) At that point you would be looking at an adjustable Fuel Pressure Regulator and possibly a custom larger Fuel Rail..