I'm not 100% sure, but I think removing the downforce would aid in top speed if memory serves, but it probably doesnt. I did have a Le Sarthe setup for the Speed 12 which hit 295, but I think I changed it, silly me.
you don't need it at all, because the more wind that gets under the car the slower the car becomes, if you loose it, then no wind will pass under the car thus more wind going over the car and ultimately the faster the car goes. Try it mate.