With the Shimano 9000 11 speed cassette and groupset having just been recently released, it raises the possibility of mixing and matching Shimano and SRAM 11 speed components. Here's what seems to be working:
If this works, then there is no reason that SRAM 11 speed shifter and derailleur combo should not work with an 11 speed XTR cassette.
What is a little frustrating is that there seems to be no real data or measurements on the Shimano 11 speed pull ratio and rear cluster gear spacing. This should be easy enough to measure, but no one is doing so at the moment.