The stock fuel system keeps a constant fuel flow as well, just less fuel. Most FASS systems that I have seen hold 55psi all day long with big injectors, that is high volume. The stock pump has less volume but still keeps a constant flow. The stock pump with the proper mods, that are required for a FASS as well, will flow plenty of fuel for hybrids. That has been proven by DI and BWD.
So, if you have $1300 burning a hole in your pocket, get the FASS. If you want to save $750, get the ITP diesel regulated return.
Keep Strokin'