Something that the old-school two-stroke bikes would recommend is to de-carbonize the pipe and muffler. You need to buy a can or two of hi-temp barbeque paint first. Second, you take and build a really hot campfire in your back yard, and then you place the expansion chamber and muffler into the center, then surround them with coals and blazing logs. I'm not kidding, it's worked great for me - Your pipe will get to 1200 - 1700 degrees and that'll burn off all tars and oils. Let the fire burn out and let the pipes cool. Then pick them out and bang on them with a stick. Once you knock out the carbon, repaint the pipes with the barbeque paint.
You'll be surprised at the difference, and it might very well clear up your top-end performance.