The only mod I know of that changes the exhaust note is the Trooper mod that is still an ongoing thread.
If I understand you right, you are asking about the Wynpro Air Induction Covers. The main purpose of those covers is to clean up the area underneath the tank by removing the bikes OEM PAIR system that recycles unburnt fuel. The claim is that removing the PAIR system reduces deceleration popping, but that is not the main purpose of the covers. The way the PAIR system works leads to the throttle bodies getting gummed up after some miles and the covers reduce this. They also make working underneath the tank a much easier job because all the hoses and gunk related to the PAIR system are permanently removed.
The only other sound related mod I can think of is CDog's air-box mod, but that just kind of increases intake noise due to opening up the airbox some.
Good luck.
