The way i see it return line adds the ability to tune. See stock injectors run at static 50+psi (don't remember the exact number) where as the 650's and 750's like rcs are designed to run at 40 psi by using a return line you can rough tune for your injectors and then fine tune with safc or other method so you don't have to pull so much fuel with the safc. Plus when you install large injectors on a stock fuel pressure a 550 injector comes out 2 a 650 and so on becaust of the extra fuel pressure. So i see it as a way to compensate and tune for your injectors
