Originally Posted by Joseph Davis

Just remember that your fuel pressure @ the rail is going to be pump pressure minus intake manifold pressure.

Most returnless setups seem to run ~55 psi. So, you get more "differential" pressure between the 55 psi in the rail and the -6 psi in your intake during a part throttle cruise... effectively 61 psi of fuel pressure. When you boost it works the opposite... 55 psi rail pressure minus 12 psi of boost = 43 psi of differential pressure. As soon as you add in the return line, expect the tune to go lean at part throttle and rich under boost.

No big deal, just don't press the returnless setup too far.