SRAM started out with the "1:1" Shimano-compatible stuff, then went off on their own "2:1", (or is it "1:2"), stuff.

Now that Shimano has changed the cable pull on the 10s mtn RD, I've been wondering myself whether the current SRAM cable pull *might* be compatible with the 10s Shimano mtn RD.
I have a new (uninstalled) 10s XT RD here, and in playing around with it, it appears the cable pull is roughly twice the pull of the 9s XT RD.
I haven't done any precise measurement, but I recall a recent posting where someone said the actual SRAM cable pull is really 1.7, and not 2.0

But something cool is that you can use these together:
- Shimano 10s road shifters.
- Shimano 10s mtn cassette.
- Shimano 9s "Shadow" mtn RD, (yes NINE speed).
I have this combo on my touring bike, and just built a new special-purpose climbing bike with the same.