Originally Posted by
Schlafen
If he's moving to 11s it will not work with a 9s XTR, different pull. A 9s 40t cassette may be available but that's not what he's asking here.
For the RX800 to work with his DA9000 he needs at least 105 R7000 shifters and a FD9000.
If he goes for 50/34 rings and a 11/36 cassette, for which the derailleur is rated as standard, he'll have enough range where he will not need to run a 40t, even though the RX800 derailleur will most likely work with a 40t cassette without needing the extender.
40t is usually used with 1x for range but OP wants a 2x so 40t will be overkill and with a 36/34 ratio he could climb up a wall anyway.
It depends if current bike and derailleur hanger, if it's a steel bike with an integrated hanger it should work no problem as they have longer hangers than the removable type.
I'm well aware that Shimano's 9-speed XTR and 11-speed road RDs use different actuation ratios, thanks (approx 1.7:1 for Shimano 9 speed and below, except for older Dura Ace, vs. approx 1.4:1 for 11-speed Shimano road if I recall correctly). If the OP goes Shimano 11-speed, he'll obviously need new a new RD as well as shifters.
Above, I didn't think I needed to say that "with his existing RD" included the use of his existing 9 speed rear shifter as I thought that would be both obvious and understood - particularly since I also indicated he'd need to get a
9-speed, 40T cassette. I was merely pointing out that if going to a larger (e.g., 40T) max rear sprocket was the OPs primary goal that there might be a way to do that without the expense of new RD/FD/shifters.