Actually, Tony, you are incorrect on the locked diff for full-sized FWD racers. Those are really only used in drag racing. Road racing cars that are FWD use limited slip diffs. These are not fully locked and are able to have some differential wheel speed. The reason for this being that a FWD car with a locked diff can be very difficult to turn, especially in low-speed corners, due to the fact that both wheels will turn at the same speed. Cars with that set-up can be a real handful to drive. Basically you are fighting the steering through every corner as the inside tire hops and skids around the turn. Generally, a limited-slip allows you to steer through the corner easily as an open diff, but transfers full power to the wheel with the most grip and a percentage of the power (based on how the LSD is set-up) to the wheel with less grip. This basically means that you get full power to the outside wheel and a percentage of the power to the inside wheel. The result is that you are able to maintain corner speed through corner entry and apex as well as launch hard as you accelerate out of the turn. I've driven a FWD racer with a properly set-up LSD and it is infinitely better than one with an open or a locked differential...
Ok, now back to our regularly scheduled programming...