Maybe the collective STW knowledge can help me out here. Is it possible to run the following set up:
XT 2 x 10 cranks
XT 10 speed front & rear mech
XTR 11 speed rear shifter
XTR 11 speed 11-40 cluster
Shimano 11 speed chain
As I understand it the 10 speed rear meach has the same throw as an 11-speed one, so this set up should work, but someone might have some rear world experience on this
From what I've read you are right. But zero experience of riding it.
Parallelogram is different on 11 spd mechs to track closer to the 11-40/42 spread. On a 10 spd you'd need to use the b-tension screw to pull the guide pulley away from the larger sprockets leading to poor shifting elsewhere.
11 spd XT 11-40 cassettes and GS rear mechs are confirmed as being due 1st week in July at Madison - just get them - still cheaper than XTR cassette on it's own.