I have the stock cat-back and a RMM DP and I hit 22psi before. I can hit 18psi+ without a problem, although the stock cat-back does cost me some power. I usually run ~113mph traps, I took the cat-back off and ran an open DP at the track once and ran 116mph traps, so a free flowing cat-back defently will give some power over stock.
The stock cat-back flows fairly well. I ran an [email protected](with a 1.68 60' time) with the
headlight in. The next week I came back with
the headlight out and a Greddy SP cat-back and
my mph improved to 119.25. Probably picked up
~20 hp from the cat-back. This was on 19-20 lbs boost.