I think it's like this
I think it's like this: have as little pipe as you can, but it has to be under 95 dB's. I know it has to be under 95 dB's for a fact to be street legal. Not sure if it's at a certain RPM--3000 sounds familiar but I can't say for sure. Most aftermarket exhausts try to reach the perfect median--as little pipe as possible for best air flow but all while staying under 95 dB's. Think about it -- NASCAR cars are loud as all hell, and they have no mufflers, cat.'s, and a lot of the exhaust is non-existant. Their HP is damn good though. It's tough to accomplish high-flowing exhausts while still keeping quiet, so as a GENERAL rule (not always true), the louder the exhaust the more HP. I don't think it's based on the length of the pipe though.