FRP is very good ay maintaining it's shape, as long as they have put some UV protectant in the gel coat - which is standard practice these days I have been told.
You just won't really be able to slam your hatch if you crack the shits I would imagine. Technically you can't use Carbon doors on a road registered car (at least in NSW) of the sprinters vintage due to the side impact ADRs that the car was complied with.