Mustang Enthusiest recently did a test with the stock H pipe, a number of aftermarket X and H pipes with and without cats. I don't remember the exact numbers, but it was less than 10 rwhp and 10 rwtq difference between any of them, and IIRC it was more like 2 rwhp/rwtq. There is an obvious difference in sound, but very little difference on the dyno.
That X pipe that you can swap out the converters looks like a nice option....