Yeah, I am not quite sure on what is happening with Shimano's MTB rear derailleurs right now. I have heard reports of changes in pull, but...
Up until I read that, I adopted Sheldon Brown's approach that all Shimano RDs did exactly the same thing, whether they were nine or 10 speed. That's why I have identical previous generation XT deralleurs on both 10 and 9 speed bikes.
If you have any doubt, look on the internet for NOS XT stuff, if you really want it. I would settle quite happily for LX or plain old Deore, frankly.