I love to hear the sound of the engine and nothing comes close to driving thru a tunnel with the sunroof and windows open.
I have sat in an M6 with CF roof with no sun roof and it feels very claustofabix. The CF roof may offer a few tenths of performance increase, but I dont care. I am not racing my car and a few tenths of performance loss is of no significance.
Also, the resale value of cars with CF room will probaly be lower than the ones with the CF roof. CF roof is something only the hard core enthausists will opt for and the market for CF room will be small when you look at the overall sales for 4-5 years.
As I am finding out, heated seats, power everything, nav, etc help immensely when you are selling your M3.