The wheels rubbing the insides and outsides of the arches can be a problem if you buy large spacers. I have an E85 and I've got 15mm on the rear and 10mm on the front. Some have put 15mm on the front but have suffered slight rubbing which they then start to mess with the arch liners to stop this problem. Theres a recent thread discussing this.
I had a set of 20mm spacers on my Z4M that were not wanted when sold. I've fitted them to the e89 to see how they look, if they fit and if they help/ hinder handling. Car runs on stock 293s with non run flats and M adaptive suspension.
First impressions;
Looks great IMO fills the arches nicely and there seems to be no rubbing or handling issues. Will post up some pics when I get a chance.
Different wheel offsets push the wheel face further out from the brake hub and fitting spacers does much the same thing. I've run spacers on several different cars over the years and never had an issue with bearings or tyre wear. If it makes a difference it must be negligible.