Originally Posted by tom @ eas
I never said this was a rule for everyone to follow, it's simply something we don't recommend. The additional shank length is only one of the factors.
Past experience can be seen on e46 M3s, which are on their 8th year of seeing US roads with no ill effects of spacers within these thresholds. If referring to e9X M3s, there's not enough time to tell, but I doubt it would be drifting far from past models. Spacers are even a common mod of Ferraris that see track duty to increase track width.
Staying under 20mm is perfectly fine, I would simply avoid going further than that if at all possible.
thanks, tom. i see what you are saying, but there still is no absolute data. for instance, what evidence do you rely on to know that over 20mm is bad?