I have Crawford hi-flow cats, won at ZAttack and Crawford headers, won at ZDayZ. I'm running them on stock exhaust, and the exhaust note is awesome for a daily driver. And there is definitely more power, although I don't have a dyno sheet to prove it. Soon though - I had a bad coil pack at our club dyno day and was missing a cylinder. But hi-flow cats or test pipes are the cheapest exhaust mod that makes any sense - $1000 for a cat back might get you 10 HP. Not worth it IMHO.