any neg effects to running spacers

The 10mm Spacer Dilemma
10mm wheel spacers are one of the most common spacer sizes out there. However, what a lot of people don't realize is that a 10mm size is not a true fitment on any BMW. It's considered a "tuner" fitment, meaning some extra thought is usually required to correctly install them. Only with the right combination of parts will a 10mm spacer fit without a vibration or other fitment issues.

Factory Hub Lips Are Greater Than 10mm
The depth of the lip on the car where the spacer will rest will vary. The available lip space may be 11mm and a 10mm spacer with its own hubcentric lip cannot physically fit. The spacer will bottom out on the lip instead of on the brake hat, leaving a gap between the spacer and the rotor hat. The biggest reason for this is the thickness of the rotor hat. The thickness varies by manufacturer and design. A thinner hat (such as used with aluminum Euro Floating rotors) will leave more lip protruding and make the problem worse. Most spacer manufacturers add an extra chamfer, or shoulder, to push the hubcentric lip further out. The extra chamfer clears any extra part of the stock lip. The other way to get the spacer to fit is to shave the stock lip with a metal file. While not the most elegant solution, it works.

JasonNW wrote

Is that true for the E90 M3? I was thinking about 12mm front, 10mm rear. Do 10mm Macht Schnell not fit right in the rear? I've seen cars that go as low as 5mm...