I would think that the high roof would be a natural for other than a roof air conditioner. As has been mentioned, it will get too high, and you have added a lot of volume to the inside, so you should be able to spare a little for a split system or a Coolcat type wall/floor system.
If this is a MB supplied item, I would not worry about safety, as the Germans tend to overdo rather than underdo that kind of stuff.
If the tall roof is on a dually 3500, they still will have the option of adding the crosswind assist that they use on the single wheel units, so I don't think that will turn out to be a big deal.
Fiberglass is more sound deadening and less heat transmitting than steel, so that should be a plus.
If done right, tall roof Sprinter could be just what the tall folks need and want, without going to a C. We have had a lot of folks look at our Chevy Roadtrek and say they liked it, but were just were too tall for it, so the market would be there, especially as the younger, taller, generations get to wanting a B.