On a stock internals motor, an exotic header and 2.5" exhaust ain't gonna make much of a difference over a pretty standard assortment of parts. The "regular" stuff is pretty capable of flowing up to the needs presented by the motor.

However, when you start getting into the motor, freeing up the exhaust alone can yield another 5 to 15 hp depending on what exactly is done to the motor and how much you free up the exhaust.