i was wondering what kind of horsepower gain i could be looking at for aftermarket performance headers. and horsepower gains for adding a full exhaust?

 

You should see 25 - 50 horsepower depending on how restrictive Your old exhaust is.

 

Depending on what engine you have a set of headers, hi-flow cat and a full exhaust (2 1/2 to 3') should give you between 20/25 horsepower. Any claim by the Mfg of more is a dream.

 

Ive got a 305 with about 125K on it. (On a 82 Camaro) I installed a CHEAP summit set of headers, dual exhaust and cherry bomb mufflers. The setup was a PAIN IN THE **** to install and has some clearance problems, but it woke my car right the heck up. Before the exhaust, I couldnt even spin my rear tires. Now, I can smoke them all day long doing a brake stand. I was VERY pleased. Think about it, an extra 20- 30 HP from a car that only generated 200 horse to start with, thats somewhere between a 10-15% improvement in power. (Power you can feel, not like that little 5% a KN air filter or slip on muffler tip all those import guys do!)