I'd love to drive this and compare it to the E85. My friend has a 2006 Z4M and we've been to the track with our cars. He let me take his around Watkins Glen for a session and the fact that you sit right in front of the rear wheels means the car feels like it's rotating around the driver. It's such a different feeling, almost like a go kart. I never thought much about the Z3 or Z4 platform, but I've changed my tune since driving his car. It really punches above its weight.
What classifies a car as a roadster beyond being a convertible? I drive an R8, and I've driven the convertible, which absolutely didn't drive like the Z4M. You sit too far forward for that, because the R8 Is more of a GT.