Not only are the door sills extremely easy to scuff, but they're actually very fragile! I found this out the hard way when I tried to peel one back to show a friend how they're velcro'ed on, and with only a slight amount of pressure, the door sill cracked! Turns out it's some foam material, covered on top with plastic. From the underside, you can indent it with a fingernail.
I think a full carbon-fiber door sill would be overkill (and expensive), but plastic with a thin carbon-fiber cosmetic layer on top would look just as good and probably be a lot cheaper. Al & Ed's quoted me $2500 for the pair of full carbon-fiber door sills.