I race the M9000s, been through hundred mile races, long and short descents, steep stuff, etc. I've been impressed by their heat resistance using various icetech rotors, but not their power. IME, they fall significantly short of something like the 785/8000/7000. IMO, these would be over-taxed on a gravity-orientated bike. The big difference is the servo wave, but it's a small weight penalty to go with something like the M8000.
This is one area though that I do not try to save too much weight. IME, those icetech rotors are the real deal and do dissipate a significant amount of heat. During long or very steep descents, this is a must for me. I could go with some ashima rotors to save some significant weight, but I have baked the hell out of those during a few downhills and apart from use on my winter fatbikes, I would never run them for the terrain I ride during the summer. There's also a winter fat-bike downhill event at the local resort, for that, I have to take the ashimas off and put real rotors on. The bike I use for that also has M9000s, but the braking requirement is nowhere near the same 99.9% of the time. I would prioritize power and braking ability first, weight second. So I take a bit of a "hit" with the icetech (and jagwire copies) rotors weight-wise. I bet there is plenty of weight to be saved on your bike that will more than make up for the difference of M9000s, like front carbon layup vs. rear layup and choice of spokes/nipples for the front vs. rear, or rear hub, etc...
Shimano is near the bottom of my list these days for brakes though, with no seal kits available from Shimano, they expect you to just trash the brakes and wait till they suddenly fail some day. They also suffer from the issue where when you don't use them for a few weeks, the caliper piston seeps some fluid and contaminates the pads. The wandering-bite-point issue is not as bad as with my XTs. Once I run out of spares, I'll probably be back to Hope or Formula. There's a good mega-thread on brakes on Ridemonkey to explain some of these issues further.
