Kaefer: Only if the dealership can prove that the modification was the cause of the failure.
If your springs go out, they cannot blame the pulleys. However, if your pulley breaks and something goes wrong, obviously since you have an aftermarket pulley, the dealership does not have to cover repairs and you'll have to take up the issue with the company I'm having make the pulleys (who is a big name in the BMW/Mercedes/Ferrari market)
Since not very many folks are feeling the pulley right now, I'll save this GB for later. I need to update my signature to add in all the other stuff I'm working on.