You won't see much of a gain, especially on a near-stock or mild setup. On a related note, you won't see much of a difference between a high-flow cat and a test pipe.
I chose a turboback exhaust that had a high-flow cat and a resonator to cut down on noise and so that I could still pass emissions without swapping the stock exhaust back on. IMO, catless exhausts are way too loud, and usually stink pretty bad (this can suck in the summer with your windows down).
If you keep your current setup, keep the stock cat and spend the money elsewhere. You won't notice any gain from it.
on the too loud and stink. i have full manderl bent exhaust with no cat, i have to shift early accelerate very slow so i dont attract the cops(btw my car looks beat up; no gloss, slow, etc. so i dont like the added attention).