Can u not try a size bigger torx bit, I m assuming it is tensioner.

If you could post some pictures that'd be great so we can have a better idea of whats going on. Also if it is a bolt on the tensioner why don't you file down 2 sides of the head of the bolt and grab it with a pair of vise grips once you have 2 flat side to grab on.. Please keep in mind that the bolt on the tensioner is a reverse thread.

Ed

Read the topic 'A tale of 3 superchargers' and note the method I used to remove a stripped SC oil plug. The same thing can be done with the tensioner.