First, don't think in terms of "what does this part give me in hp", because the engine works together as a package. Each part is a peice of the package, and each peice will work differently depending on the rest.

So a catback exhaust by itself is not going to do alot, but when paired with intake/header/cat, its going to provide good gains as a system.

A high quality proper i/h/c/e setup can get you anywhere from 13 - 20+ whp depending on the engine package and what you go with. (all DC sports vs Hytech/SMSP, obviously the latter will produce better gains)