I would not recommend changing it, spend your money elsewhere. You can get a little less back pressure with other pipes, but it only changes the power curve, it doesn't really help. I have never been in the top gear in a race, which tells me that low end is key; If you open the exhaust up any more, you are going to lose critical low-end.
And no, the stock pipe is not quiet and nowhere near street legal. If you are one of the idots that think a louder pipe helps, try to learn how these engines really work and the reason for back pressure. Most of the people that do that are simply trying to draw attention to themselves at the track or on the street.
Seriously increasing the power range is only going to shorten the life of the motor for a very small improvement. That may help the top level racers but few people can ride these bikes to their potential, and certainly nobody on this site.