a tv show i was watching about go karts showed a kart stopping within 10 meters because he did this weird spin, but i don't think i would want to try that on a normal road, especially in a car, i'd be worried about rolling it.
it won't because the master server doesn't know your internal network scheme... it only affects people who connect to your server from your own network, and nobody else.
now there is a way around this, but if you're using a store-bought router, it won't be possible... and that would be to send any outgoing traffic destined for yourself back into your network. (edit: unless this is what amp was talking about)
the maximum for the per-connection (carshost/carsguest) settings is 3 (or was it 4)...
besides, the cpu power required to make all the ai calculations would crush the system if you were allowed to add more, which would make the connection lag, and if there were 15 lagging cars on the track, it would just be a complete mess.