Most offroad pipes i've seen, dont have cats, so therefore you would need o2 simulators to "trick" the computer into thinking that the car had cats, and were functioning. And if you're going to run w/ an offroad pipe, why keep the stock mufflers? Or, why keep the stock headers? If you dont have the $$$,for it all, i say just go w/ a catback system (if you dont already) but if you've already got one, then do the Xpipe. Long Tubes are another good investment, but again are $$$. you can always wait and put each on together...but whats the fun in lookin @ parts that WILL be on the car vs ARE on the car?
As far as putting it on, i would assume you'd have to unbolt the header flanges, and then cut the existing Xpipe out, and then put in the new pipe. But it'd probably be better having a professional do this, being they have tools and a lift and all that, but more power to ya if you do this yourself.
Try going to the portal area, or to one of the LS1 sponsered sites, (LS1tech.com and LS1.com are just two...) and visit some of their sponsers, or visit the Portal area, and just search around for exhaust. I wouldn't be suprised if Dr Gas had a C5 Xpipe. But, Bassani and mufflex i would bet have an offroad X.