It will be a noticable gain since your engine combo could sure use a good set of header's instead of those stocker's which aren't bad for a stock engine but a restriction on an engine like yours. Possibly a 20+HP gain could be in the work's with both the header and exhaust install.
You will see a difference no matter what, definetly more then what a stock engine would see in gain's as your combo is more in need of a good header/exhaust set up.