Ye I see your thought process, its just too Audi.
The cs on the back does look cool, but not so much with the M logo. Im in two minds, I like the idea but like I said earlier I think I would design one myself and get some cad mock ups made, then get some machined. Or just leave it as is. You could mock up an M style logo but instead with an S that continues the form of the logo italicised with and I from an OEM BMW logo. Using the same colour combination as I think it really works. Its the fine details, the font and the way it is placed within the logo doesn't scream BMW and is in my mind unsuitable (especially on such a good looking car). A custom well made one with a suitable font and attention to the form of the BMW M brand would really look the business. Not only would it be in keeping but also very custom and unique on a BMW.
Sorry the graphic designer in me... I get really anal haha