I have found that a loud exhaust, legal or not is a cop calling stereo system. One can get away with a bit more speeding and agressive driving if the car is quieter. This does not mean it needs to be as quiet as your grandma's Acclaim, but sensibly quiet.
I would rather be quiet and speeding than loud and hassled by the fuzz.
That being said, I am amazed at how guys with Harleys can get away with more noise than ANY car around!