Sounds like OP just had contaminated pads--an issue already resolved.
I do find that with mineral oil based brakes, it's much easier to contaminate the pads. I always remove the pads on shimano brakes. Adds like, 1 minute to the process, and the only reason I wouldn't do it is stubbornness.
As far as replacing the rotors--typically not necessary, however, if there's a significant amount of discoloration, shimano will tell you to replace them. Really, soaking them in isopropyl and using some scotch-brite will do a very good job of cleaning anything simple contamination would cause.
Bringing back pads: really, I find its a matter of how much they were contaminated in the first place. Soaking in isopropyl, sanding, resoaking, sanding, then braking into them until they are very hot, then flushing with water seems to be the best method. I find power never really gets back to 100 percent with mineral oil based brakes, but I've had good luck with dot contamination.
Last 2 cents: unless you're obsessed down to the last gram (in which case, theres MT8s and formula R1s), xtr brakes are the best on the market in terms of power, modulation, ease of service. I've had 10 times the headache from Sram's brakes than Shmano's. The new style are better, but XX brakes have a very solid history of feeling like crap and requiring constant attention. However, there are some problems you want to watch out for. The main ones I've seen have been the banjo crimp-on leaking, and some of the ceramic pistons not holding a seal (probably imperfection in the way they were cast), and over time weep mineral oil out behind and onto the pads. If your brakes have a tendency of getting contaminated, take the pads out, clean the pistons really well, then check back in a couple of days to see if there is any residual mineral oil. If so, that would be a warranty issue.