T-Rex is 40t, but as there is an 11 speed 11-42 XT cassette, I see no reason why it wouldn’t work on 10 speed 11-42 as well.
For comparison, I was running a Zee FR mech and I thought that was good, but the XT GS mech is loads better. Upper jockey wheel looks like it’s placed very similarly to a 10 speed mech with a Rad/Radr cage.
Next on the list is the proper 11 speed cassette and shifter – will try other conbinateions when I get them.
Another thought – did I read that an 11 speed SRAM mech will work with an 11 speed Shimano shifter (or Shimano mech/SRAM shifter)? If so, 11 speed Shimano mech may work with SRAM shifter, if youre are sufficiently deviant enough to use SRAM.