Yea mine was down on power after decating much more drivable after putting the cat back in! Prob due to the change in the delivery of torque! But with the lack of experiment on the n/a it's all about trial and error! As for the original question mine is running a one of 4-2-1 mani with a custom stainless cat back which is 2" if I remember correcty with just a back box!