Hi,
I ran into this Youtube video to autogefuehl comparing the M550 with M5C. Notice the seats in that M550 at ~8:15 sec. They look exactly like the M5C seats but it sounds like they are leatherette, they still look good and make the car look more like an M5 but with the softer suspension!
When I looked on the BMW website as I'm building the car, I couldn't find them even though the guy in the video states that these seats are only available in the US.