I’m looking at getting a hope T. Rex but hope don’t suggest it’ll work with a 11-34 cassette. Can anyone explain why please before I call hope as I can’t afford to replace my xtr cassette just to get a 36 cassette. I’d like to use it with a zee mech too

I use a homemade 40t with a zee, shifts fine from a 36t, might be a bit slower with a 34 but 6t gap should be ok if you are not too worried about lightning fast shifts.

http://www.pinkbike.com/video/351634/

Only works with a 36t.
The jump from 32 or 34 is to big to transition smoothly plus the ratio jump on say a 30/34 to a 30/40 is Larger too most jumps are 2 to 4 teeth on a cassette.