R2 Bike lists the M-987 caliper and levers at 380g a pair and the M-985 levers and calipers at 450g a pair. The M-985 weight certainly corresponds to my pair and dropping 70g with the new version is very tempting. The only weight saving over M-987 is Formula R1 racing which drops another 20g.
I would love to try XTR M-987 but can offer my thoughts on M-985 and R1's. R1's lost power reasonably quickly on the steep, technical Welsh descents I rode as the brakes overheated even in the middle of Winter causing the pads to glaze and have a low friction shiny surface- not ideal! This was with lightweight rotors which contributed to the issue no doubt so I had to run the stock Formula rotors to get anywhere near decent performance although a bit of overheating came in on repeated longer descents. I have been running the M-985's in a comparably light overall configuration by using lighter rotors and not suffered the same issues so the M-985's appear to deal with longer descents better than R1's in my experience (even XTR+light rotor vs R1+heavy stock rotor). I would have thought you could be happy with either for XC racing where the descents are not so long or technical so the overheating concerns of the 987's shouldn't come in to play I wouldn't have thought.
Bottom line is that the M-985's have been truly fit and forget and have given consistent and reliable braking in all conditions even with super light rotors... but then I guess you already know that. I would be very surprised if the M-987's were noticeably worse as Shimano have such a reputation for performance above all else (forgetting the Ice Tech rotors which delaminate on Alpine descents
) and I would be very surprised if you needed to run them with Ice Tech rotors to get the right level of performance out of them. Personally I would buy the 987's if I was buying now but that is more based on trust than experience. Looking forward to any WW reviews of the 987's.