In an old issue of Mopar Muscle the stock manifolds cost 80 HP on a 500HP motor. I think with a full exhaust the differance may no be that much but a GOOD set of headers will alway make more power. Some FAST cars have cams with wide LSAs to help make power with the restrictive exhaust manifolds...Most of the faster ones have higer compression like 14 to 1 and run on race gas only. I really like the stock look of these cars and there amazing performance but the faster ones are not every day drivers and are far from stock.