I know you say you are not ready for headers, but they will make a much greater difference than the cat-back with just the stock manifold/cats. Combining headers with a aftermarket exhaust is really one of the best ways to unleash the beast without spending big $$$.
As to the x-pipe it will add a few hp but without the headers it will not make a big difference. Depending on your state emission inspections you might also consider gutting your cats as a cheap alternative to headers, but it still will not be as good as adding them.
