According to the Kirkham website, the aluminum used for the Kirkham is thicker than the originals (0.059" versus 0.050"):
http://www.kirkhammotorsports.com/faq/index.html#024
Honestly, it's always a concern, and anything that's a projectile from the freeway will damage virtually any car. Fiberglass is not without its own concerns as well.
I had an large empty cardboard box come off a truck that I was following on the freeway, some time ago. It bounced and hit my daily driver and it made a small indentation.
If the unfortunate does ever happen, then aluminum can be fixed just like metal or plastic.