After getting solicitors involved, BMW agreed to fix mine at their cost. The fight initially was going bad for me until I got legal advice.
Of course messing around with ride height, wheel sizes etc, it ill put more strain on the subframe, but again it will just cause the damage to happen sooner. As I mentioned, mine was bone STOCK when I noticed the cracks. As 02PRUV has mentioned it affected a wide range of models. Not just M3's. Ive even seen it in verts, and ive personally seen a 325 that had bigger cracks then mine on the host :/
What I CAN confirm is though, if your car is less then 10 years old, you can force BMW to fix it at no cost. - However, you might need to get a solicitor to do the fighting for you...
What I cant stress enough is what Alan said, if cracks happen they will go from tiny to huge in a VERY short time. Its a good idea to have you mechanic to check it out at every service, and don't just check your rear mounts either, my cracks were on the front 2 mounts where you need a cavity camera to get in there to have a look.