Check this out:
http://www.6post.com/forums/showthread.php?t=968941
This guys has a 640i GC and installed the 433M whiles which are 0.5" wider than 343M you are talking about.
He claims that sometimes rear tires rub-off with the car underneath plastic stuff. Since 343M are narrower, you may be fine, but it's better to check with your dealer first. For sure they know better than us