It depends on how often it's attached to the chassis of the car. The fibreglass floors on our Ginettas are only about 3mm thick and withstand the diver climbing in and out with no problems, including the heavy mechanic stomping around inside it.
One thing to consider with vehicle floors is weight distribution. Is it an actual competition car? If so are you on the minimum weight? If your already having to add ballast, then leaving the floor as is or making a heavier floor is better.