Its all about the warranty. If you change the pulley and something goes wrong, you have an expensive problem. I work with a guy who's son has a supercharged Saleen S281. Beautiful car. He collapsed a piston and took it back to the dealership for warrenty work and they basically tld him to get lose. There was wrench marks on the nut, and for those who have considered changing the pullies on a factory supercharged motor, Ford or at least Saleen, puts a dobb of what looks like silicone or something else on the pulley and the shaft. If this "dobb" is disturbed in any way, they'll shaft you on the warranty work.
Caymon