racerbruce - I haven't figured out BMW fitment yet, but this was the product that was very popular for lowered B13 Nissans. Lowering those cars (Sentras, Sentra SE-Rs, NXs) came at the expense of travel, so these bumpstops were used to mitigate bottoming. They are available in different compression profiles and trimmable - an interesting product for the nissan world anyway, not sure how they compare to OEM BMW.
Koni Bump Rubbers (sounds dirty!)
Product on T-Dot