I'm just on 18inch amg wheels no idea what type they are on every car I've had spacers have improved handling but they do normally look pretty good tooWhat are you wanting to fit?
The AMG 55 Winter wheels are ET37 all round, so you can definitely push the fronts out 10mm and Lorinser run their 20" x 9.5J rear wheel for the SLK at ET26 which would imply you can push the rear out by 9mm more than standard. Brabus sell their 19's in 8.5 and 9.5" front and rear, both ET35. 15mm might be doable, but that's ET22 at the rear, which is a huge increase in track. I'd love to see what that would look like!