I agree, there is a thought that a coupe is faster just because it is a coupe.
I would guess that if you put an M5 side-by-side with a Camaro or Mustang, and not even a high-performance version, most Americans would pick the Camaro or Mustang as the fastest car. I may even say that a strong majority would pick the Camaro or Mustang over the M5.
It goes back to a similar way of thinking as stickers on cars. I am sure they make some people think there is more performance there.
I have experienced this myself with a guy in a 325ci, I guess he thought that his coupe made up for a 50 HP difference. After all, mine only had 27% more HP.