I've got a 97 GS and I was wondering how much hp i can get if i put on a test pipe.

A test pipe should give you an honest 10-15 h.p. I thought about it myself but it makes your car really loud plus cops listen for that stuff around here. A friend of mine straight piped his cat and got about 25 h.p. to his gs-t. Unfortunately for us hp doesn't come that easy for the nt.