I've recently installed a kit from Ritter Cycle Racing (RCR) that included a pipe, air filter, main jet, and Primary spring. I also asked him to send a secondary spring (extra $20). Total cost was $330 including shipping. It changed the bikes personality. Now it is much snappier. My friend can still beat me in a drag race on his 400, but by only a couple of quad lengths. I feel it was a great improvement for the money spent. Only wish RCR sold a quieter silencer as it makes almost as much noise as my motocross bike.
Scott.