RobIII, anything you can do to reduce friction is going to help. Curved risers, exhaust porting or headers all help the engine breathe better. The horsepower your motor can put out depends on alot of variables; compression, cam selection, ignition timing, carburation, fuel and on and on. Your exhaust system doesn't define the amount of horsepower your moter will produce, it only adds itself to the larger equation.
Good Luck! LDFTRBT