I think you should just get a moon roof and not worry about it too much.
Also, expense aside, carbon fiber can be a very lightweight but very, very strong material. It's used in exceeding quantities in many fields nowadays, not just a car's roof, but Formula 1 (think crashes and survival), safety devices themselves (HANS devices and even whole helmets structured in carbon fiber), vehicle monocoque shells, aerospace and flight, even medical uses. Also remember that the car does clear the required safety testing in most if not all nations in the world, which means, of course, Germany's and Americas. I would think that the material strength on a 70,000 dollar car is going to be higher than, say that of a Scion trashCan's....and those clear mandated safety requirements.
I think life is too short to worry about your roof. in a roll-over crash that is never going to happen unless you turn off traction control in the rain or with bald tires, or go racing in the canyons or go on a "Mexico run". Knock on wood, but roll over crashes don't just happen when you're at cruising speed minding your own business on freeways, or happen when you're driving 45 in stop and go traffic in cities. So, unless you're a serious offender of society's imposed safety limits, I'd be more worried about your plastic fenders and front bumper.
Oh, also, FYI, old E46 M3's (2002 era) front bumper supports were made out of carbon fiber. They were replaced with non-CF ones in subsequent years due to cost reduction, but if the front bumper support of a car is made out of it, and many more front end collisions happen than roll-overs, then yeah, I wouldn't worry.
Fun story, a friend told me that shady body shops used to take the CF front bumper support of older cars off and sell them, then replace them with the cheaper OEM ones, when these cars would go to cheap body shops after being involved in a collision and the front bumper support was okay.