An interesting idea, but remember that a car takes a lot more physics and rendering processing, not to mention memory, so you'll never be able to add 512 cars instead of hay bails, and I'd imagine that for every car you added, you'd be able to have one less person join the server. There's also the issue of the dedicated server, as I don't think it runs the physics, so either it would start having to, or somehow get this offloaded to one of the guests.