Any SRAM 10 or 11 speed rear mech work with any SRAM 10 or 11 speed sti.
I run a 44t hope narrow wide chainring, sunrace 11-42 cassette, SRAM apex sti and x9 medium cage clutch mech. It worked flawlessly during the dirty reiver on my whyte saxon cross and has continued to do so since.
It easy to remove the redundant shift lever from the left hand sti as well, although I am considering puting it back in without the pawl so in can use it to control a dropper seat post.