Originally Posted by Doc
I think that BMW added the carbon roof to the coupe only for marketing reasons. Performance benefit in terms of rigidity an weight are not significant between the steel roof and carbon roof with the rigidity of steel matching the performance gain of lighter carbon. The sedan does not have it since its the family option and most guys choosing it want more luxury features such as a sunroof.
improving the handling is a marketing tool for the M3. small things all add up, move the calipers inboard, aluminum subframes, CF roof. it all adds up - any small part on its own is not significant. but together they make the heavier e92 handles better.