300 MPH in a "street car" would definitely be interesting. I have been talking with a couple of ex-NASCAR guys who will soon be going after the world land speed record for a piston powered car. There's a lot more going on to do this and live to tell about it than you might think. For sure, you want downforce so that you don't take off, but too much downforce robs horsepower significantly. It's a fine balance.
Just last week we finished all of the surfacing for one of the cup cars in the 2009 NASCAR circuit. As NASCAR is now significantly limiting track testing, more teams are turning towards virtual Computational Fluid Dynamics (CFD) studies to understand air flow and downforce with various suspension set-ups. Computer simulations help, but still not perfectly matched to the real world.
I'm with Shelby, no way are the "boys" are big enough for these kinds of risks!