All of what Todd said. As far as sound, I'm real pleased with the x-pipe included in the Kook's system I have. It sounds "low and rumbly" to me. I don't seem to notice any high pitched NASCAR sound, but then again a lot of decibals have gone through my ears. I did the Kook's upgrade well after my tune (80K miles maybe) and had nothing but positive affects. If you're looking for maximum hp and torque gains, do the whole system, headers on back. When you do it, go with one that has been engineered to work best together, or go to a reputable custom exhaust shop. Don't pinch pennies and try to mix and match an exhaust system. You'll probably be disappointed. BTW, I do know of a great custom exhaust shop.
300A
born 08-27-02
Adjusted air in and air out.
400+ timeslips.
When in doubt, GAS IT! This will either fix the problem, or end the suspense.