I had a DMC quiet on my LTR450 and it was pretty quiet and still gave good flow. There quiet version is a little longer to allow for more packing.
HMF's new competition series gives you the option of 3 tips. 1 small for quietness but only a little bit more power, Medium for about decent sound and decent power and open for lots of power and sound.
I don't like loud pipes myself. I'm sticking with the stock exhaust for now but may get a slip-on for racing down the road. When racing it is nice to have a little bit louder exhaust to you can rev the motor to let a racer know your behind them and get out of the way.