This seems like quite a sensationalist and delusional issue to bring up.
For the record the 3 series doesn't have problems in crashes. They are the 3rd least likely in their class to result in injury from hitting a telephone poll at highway speeds. With the only damage being injury to lower left leg, ankle and heel, not severe injury but just injury meaning broken bones. That's very good considering the competiton's results and the fact that you're slamming into a telephone poll at highway speeds. To potentially limp away with broken bones in your lower leg and no other injuries to other sections of the body according to the test from a crash that would kill people other cars isn't bad at all.
Regarding the carbon roof and crashes. The roof is in no way shape or form a crash structure. The regular metal roof is sheet metal so thin that you could easily bend it with your bare hands if you removed the roof pannel. You can even bend the roof with your bare hands while it's on the car and it will pop back into shape. The roof has as much to do with the crash worthiness of the car as the floormats.
Despite the fact that the roof material does not matter at all in a crash if anything the carbon roof is substantially stronger than the regular roof. The roof is made of either DX 54 or 56 low strength steel and is 0.5 mm thick.
The carbon roof (correct me if I'm wrong) is made of 3 layers of 3k 1x1 carbon and 1 layer of plastic with a polymer binder.
If we disregard the plastic layer, and just concentrate on the carbon fiber matrix and look up the material strengths of a standard 3 layer carbon matrix material vs the specific type of 0.5mm low strength steel BMW uses in the roof we find that the carbon roof would actually absorb at least 43 times as much enegry as the steel roof before deforming and similar amounts of energy before being pierced. Again this very rough estimate is on the low side not the high side.
So:
Q: What's the price in terms of crash worthiness of the carbon roof?
A. There is none. Quite the opposite. It's about 50 times stronger.
So really you should be asking yourself if it's worth having a flimsy extremely dangerous and life threatening steel roof over a carbon roof that's 50 times stronger.