SLP Exhaust question, Xpipe or H pipe

I am getting a SLP Loudmouth Catback for my 2003 GT. But I have a few questions. FIrst, what kind of performance should I expect? Then all this talk about H Pipe versus an X pipe... should I get an X Pipe and what kind of performance (and sound) performance should I expect? Third, drawbacks of offroad vs. legal X pipes? And lastly, would I need a chip for a legal X Pipe? Thanks for all the help.