well bolt on parts never add TONS of hp. Intake, header, and exhaust are you're typical "bolt on" parts. And a cat-back will add a few hp might be slightly noticable. It will add more than just a muffler will because it replaces all the piping from the cat-converter all the way back. I think usually you can expect about 3hp from an exhaust I could be wrong tho. I went with the Apexi WS II because it's not RICEY. If what you want is quiet while crusing and normal driving but loud and deep at full throttle the Yonaka cat-back is good for you. It's a clone of the Apexi.