matlough, be very careful. If you test drive an M135i you will end up buying one!
Nicktyler, regarding warranty, have you ever seen or heard of anyone who has had repairs under warranty refused due to modification? I believe that would be unlawful, since BMW warranty covers manufacturing defects only, and they can't be caused by anything the owner chooses to do with his car.
Warranty invalidation is a fear tactic, like an MOT station telling you the brakes are dangerous, but they can fix it cheaply.....this kind of thing is happening a lot, and not just the car business!