If people waited for the NHTSA to determine if they'd buy a vehicle or not, we'd all be walking. Are you planning on making it a habit of driving over large metal objects at high speeds? If so, buy a 4x4 or a tank - whichever fits better in your garage.
But seriously, not sure what you're concerned about. A couple of people ran over large hunks of metal at highway speeds that caused tons of force against the bottom of the car and pierced the battery pack. Shortly thereafter the car told the drivers to pull over before it shut itself down. Both drivers exited the cars and then a small portion of the large battery pack began to burn, venting the fire forward and toward the ground away from the passenger compartment.
You have seen the pictures of ICE cars with the floorboard pierced by metal objects, right? And you know that people have been impaled in their ICE cars (and some have died) via objects coming up through the floorboard, right? And you know that you're driving around with flammable fluids in your car at this very second, right?
Lastly, the KBA has already done and finished their investigation and found not a thing wrong with the Model S.