Barely any... you guys toss around these 130 and 140 numbers like its actually getting to the floor, which most likely it isnt.

say 4 hp at the most.. which he wont notice, only a smooth power band. He'll also notice a drop in low end power, and might gain a bit in the top end.. other than that he's just making his car louder, and the exhaust out the muffler dirtier..

If he still wants to do this, he's going to have a CEL, but that can go away with a non-fouler to trick the ECM into reading something decent from the O2 sensor..

or he could just not do it at all..