I had sourced new motor mounts when I ordered the parts for my driveshaft project and replaced the transmission mounts at that time. The M5 is interesting because it uses different designs for the left and right motor mounts. Based on my research, some people suggested using two of the passenger motor mounts, as they appear to be more robust in their design. It seams reasonable when looking at them.
When I went to remove the old mounts, I found that they had indeed each failed in their own way. The passenger mount was cracked not only on the metal flange, but the rubber had also separated from the metal. The driver’s side mount failed in a simpler, though equally catastrophic way. The metal part had completely separated from the rubber part, rendering the mount only able to support the weight of the engine, but not provide any support when rebounding.
After looking at it, even though I had two passenger mounts on hand, I decided that the BMW engineers chose these mounts for a purpose and that I would replace the driver’s mount with the specified part rather than implement a hack.