Actually, weight is extremely important. The air effects create a downforce at speed, allowing the car to be lighter. Light weight is what's important when in comse to accelleration and braking.
While I don't know what the ratio is in the case of 4 wheelers, but when it comes to motorcycles, every 5 pounds of weight eliminated is roughly equal to increasing engine power by 1 HP. So, if I were to go motorcycle racing at 200#, and my buddy Shane only weighs 150, when it comes to accelleration, if we were on the same bike, he would have roughly the same advantage as an additional 10 HP. And braking is quite a bit easier on lighter bikes as well.
Mark
Actually ground force holds the car to the ground, keeping it from becoming a temporary and uncontrolled plane. Ground force is your friend, it's one of the interesting battles of racing, the higher (lighter) a vehicle rides the faster it will go, but the lower (heavier) it rides the more control you have over it. Race cars now generate ground force that is multiples of their actual weight, this is necessary at 200 MPH to avoid tragic accidents, so 5 pounds here or there just isn't going to make that much of a difference.
Yeah when you're dealing with the level of horsepower of your average street vehicle 5 pounds can make a difference, 1 HP plus or minus is big. These are race cars, they scoff at the HP of our street vehicles. And of course in bike racing you're dealing with tiny tiny vehicles, many of which weigh less than the driver so obviously the weight of the driver matters much more since you're talking about a significant fraction of the overall weight of the vehicle. 50 pounds is not a significant fraction of the overall weight of an IRL or NASCAR car.