I've been reading about these two things on DI's website and from what i can tell there isnt alot of difference between the two except that the return kit feeds the back of the heads also giving you more balanced fuel flow. It also alows you to reach pressures up to 75 psi. However, you can do the same with the shim kit, increase it from 5psi to 25 psi over stock. CSIPSD on here is running a factory fuel system with only this mod, and is feeding stage III hybrids like this. So my question is, why spend the 450$ on the return kit when you can spend so much less on the shim kit?? All comments welcome