You can always get something like a Walker high flow muffler that has dual tips. Its a nice muffler with a nice tone which is not too loud, but may not be enough fo rmost people.

On any exhaust system the key will be a high flow muffler and mandrel bent piping, but even with both of those dont expect huge gains, and expect some low end grunt loss as well in exchnage for power up high in the rev band.