If you have a torx wrench set (star lookin ones) ive had luck extracting stripped set screws before, just find the one that goes in the farthest and fits the snuggest and THEN start trying to loosen it. dont start spinning things in there until you find something that fits at least somewhat snugly, or you'll just make it worse. Or if theres a decent amount of said stripped screw sticking out of the gear, then use a dremel and a cutting wheel to create a slot in it so you can use a flat head screw driver to loosen it. if you're not handy with a dremel then have someone who is do it for you though. dremel+tight spaces+inexperience=bad.
+1 on using a torx wrench, works for me every time.