After market pipes...
I would reccomend finding a "tuneable" pipe, like a White Bros. C-series, or Supertrapp. They have removable disks that allow you to adjust noise/backpressure to suit your needs. Fewer discs equals more backpressure and less noise (stock engine). More disks equals less backpressure and more noise (modified engine). You can also often get by without re-jetting just by adding/removing disks. Lastly, if I am not mistaken, all the pipes of this design are considered to have a "spark arrestor"(environmentally friendly) regardless of the number of disks (due to a closed end-cap, I suppose). Check before you buy (if legality is an issue).