An M3 convertible is already a compromise, and the extra 440 pounds may be too much for the M engineers. With the new M3 significantly faster than the previous model, BMW is currently working on a special softtop to handle the higher speeds and maintain low NVH number, despite the higher speeds.
Another option is would be to offer a carbon fiber version of the folding hardtop, which could use carbon fiber for most of the panels and the arm extensions. It wouldn't be cheap, but it would meet the M Group's low weight requirements, and offer a higher level of interior noise reduction. And it would be very cool.