I know this was a topic of interest here, so I wanted to leave some information on the subject. I just watched the M4 head to head. With the carbon ceramic brakes option and an $85,000 sticker, the M4 outran the Mustang GT PP on the road course by a whopping .59 of a second on the same track and with Randy Pobst driving. Add into that the frail nature of a track-worked BMW, and I'll take the Mustang.
I think it is safe to say that a large % of BMW owners will not ever track their car..