The chassis is weak around the area where the rear upper shock is attached.. It is a common failure, it does require welding to repair or replace the reinforcement plate from BMW. BMW has an upgraded part that is thicker.
Part numbers are 41 14 8 152 997 (left) and 998 (right). There has been a lot of discussion on this, so check the archives on Roadfly for more information. Some people do remove the old reinforcement plate before attaching the new one so technically I guess it could be said the tower has been replaced.
Stiffer suspensions systems will definitely increase the likelihood of rear shock tower failure.