I still can't seem to get the need for a CSL out of my system, might be all change come spring.
I know you can buy CSL carbon roofs for normal M3s, usually done to get rid of a sunroof on a track car - I wouldn't envisage a problem insuring it, as it isn't structural, the integrity is in the frame. Obviously you'd want it fitted by someone who knows what they're doing though.
You'd be brave cutting it off!!