They can only void your warranty if the install of the part/s is directly attributed to the failure you are requesting warranty work on. If you are claiming a steering issue and BMW attributes that problem to be linked to your install of coil overs , then they can deney coverage. Keep in mind all other warranty coverage remains in effect. They can not, by law void your entire warranty because of any installed aftermarket part. Here is a long read but sums it up.
http://en.wikipedia.org/wiki/Magnuso...s_Warranty_Act
Just realized you are in Germany. Check your local laws.