I had the same issue on my e39 when I went to replica m5 wheels.

Considering the amount of balancing they have done on the wheels and tyres and the fact that they mounted 18" and it went away. I'd suspect it has something to do with the bore. It shouldnt matter what suspension you are running thats just a dealer excuse to get out of investigating further.

Try running the 20" up front and your standard 18" at the rear and see if that has an issue. At least you can narrow down which set is having the issue. (rears are the ones that were bored correct?)