[aerohead]

In a past Cycle World Magazine article,a 'SMART' exhaust system was discussed for bikes.
The engines were operated on a dyno test cell and every conceivable transient engine load was analyzed.
Exhaust back pressure was altered throughout testing and a 'map' was developed for performance.
A 'valve-by-wire' (if you will) computerized system was developed in which closed-loop logic modulated a valve within the exhaust pipe ahead of the muffler,to always keep back-pressure in its sweet spot.
Back pressure is mandatory for low speed engine performance.Factory systems are 'tuned',and an alteration to the exhaust should be accompanied by an alteration to the induction.
I've run a couple of vehicles at Bonneville with bone stock powerplants and they do just fine at sustained WOT operation.And they get great mpg.Quiet too!
My opinion would be to leave everything under the hood alone until you've reduced your road load to a minimum with aero and rolling resistance.This is where your real savings are hiding.