It will make a tiny tiny bit more power, but then your exhaust will sound hollower, and be louder. I punched my bro's old mazda 626 years ago, and we couldn't even tell the difference besides the exhaust getter way louder then before and our car smelling at stop lights.
Honestly, it isn't worth the 1-2 hp. If you want the same effects, while keeping it legit, get a carsound cat.