I just bought a 2001 GT. It is currently bone stock and I would like to do some upgrades. Of course, money is always a problem, but I was thinking of adding Flowmaster 40s, (not a catback) and a K & N cold air system. What kind of HP increase can I expect from these mods?

 

Welcome to the club and the forum.
At a guess I would say 10 - 15HP. I would go catback though for more noticable gain.

 

Ditto.....save those pennies, you'll be happier with sound and hp results.