If it's quiet, it has the wrong pipes.
A nice pair of Epco Stainless or Thunderbike mufflers and a jet change will do wonders, especially if you pop out all the bits of rubber tubing you find in the air box. Some have 3, some only have the one at the inlet. Also, the idle mixture adjustment is critical since the idle and transition ports come back online at hight speed acting as a power enrichment circuit. Adjust for best idle speed and you should be alright. Richening the idle mix enough to stop the pipes from popping on deceleration will cost you some 5 to 10 MPH of topend. With these mods, my '03 would run off from the OL's 88 inch Lowrider and the 96 inch Roadking she traded it in on.
If you still have the air injection system (silver tubes next to the sparkplugs) installed, go to newbonneville.com and get the removal kit. It won't do anything to the performance, but it will make the pipes run cooler (less discoloration) and maybe extend the exhaust valve life.
