I've had a couple of new ring & pinions cryo'ed and REM'ed. I haven't been able to do back-to-back comparisons, but my research convinced me that it is helpful. My current gears have lasted longer than expected.
I've only done it to new gear sets. I figure starting new will provide the most for the investment. But going on my differential guy's suggestion, I have him set them up before they get sent out for the treatments. According to him, it's much more difficult to read the pattern for set up when it comes back with a mirror-like finish. The process doesn't change clearances.
I use Evans Performance in Georgia. They explain the benefits better than I can.
http://www.evansperformance.com/home2.html