In my experience:
XT M785 and saint M810 are on a par power wise
XT M775 aren't as powerful as the M785.
XT M775 are more powerful than saint M800
Plenty of people have run XT on DH bikes without a problem (Dundog and Shredden, do, I think), but I think they are the current M785.
I currently have full M775's on my old Dh bike, upgraded from the old saint M800. They shit on the M800.
That's just my experience mixing and matching/testing.
M810 levers haven't been ridden, just fitted to M775 hoses and callipers. Callipers might have a couple of marks (can't remember), but haven't had much use. Ask around, maybe. They mightn't have the power you're after. I haven't tried them for fade on long fast runs.