I just modified my stock pipe.
as you may know the stock pipe looks alot like an expansion chamber exhaust. unfortunately from the factory it is VERY restricted. The header tube goes all the way into the middle of the pipe/chamber and there is a huge baffle/restrictor right in the center of the chamber.
I've attached a crude drawing of the pipe. I didnt take any pictures sorry.
Basicly I removed the length of header pipe that extends into the chamber along with the baffle in the center. I had to cut down the center line of the chamber and reweld it back up. I also had to reweld the header joint. not a big deal really. After doing this the pipe becomes an actual expansion chamber, I can feel the powerband which hits around 3500RPM on the laguna tach. a slight bit of low end was lost but nothing too bad.
results: 38mph with the stock 12mm carb and my 220lb ass on the bike. I cant wait till I throw a 15 SHA on this thing and see what it does.