width has nothing to do with AWD, traction control or any other systems. its the rolling diameter that needs to be kept in check. as long as you keep the overall diameter identical or very nearly identical front to back, you will have no problems. with only 1/2" wider rears you can still run the same tire sizes front and back and its absolutely a non-issue. i'm sure there is also a stock tire setup that has wider rear tires that is also usable.
straight6pwr's post in Style 162 Wheels for 330xi was marked as the answer