I agree with HRC4EVER, it's hard to beat the stock muffler. Aftermarket mufflers cost at least $200, or roughly $100/HP. Not a good enough for me. Modifying any four-stroke is expensive and that's why my 400 will remain bone stock.

The best feature of the stock muffler is that it's quiet! With the greens running around, slowly closing riding areas all over the country, we can't afford to give their cause more ammunition by using offensive (to them) exhaust systems. However, I will not sit here and say "everyone must run stock exhaust." That is your decision, and none of my business. I'm simply asking that you weigh the consequences, and make the decision that's best for your situation.