X Force is one issue, never heard a good one.
vr304 beat me to it, long tube 1 7/8" 4-1 headers will increase you peak without affecting torque too much and will make it sound a lot better.
You can spend the money on the X Force to get it right, by that I mean you get rid of the X merge up front and fit a H. Long hotdogs in the middle and larger can rear mufflers. Or you can spend about the same on a decent cat back, but if you go down the 4-1 header road and need cats anyway, I do the full bolt on system of 1 7/8's, high flow cats an Manta 3" for $2950 shipped. About half a day to bolt it all on and you will not only see an increase in peak, you will also get a much better sound.