To answer a few question that were not addressed in the previous post so not to be redundant. There are no wind tunnels that can properly give the information a designer would need. You can make wind tunnels and watch the air flow over the body but obviously not the same as the costly wind tunnel test used for full size race cars.
Most bodies are designed by trying to match concepts used in full size race cars. If you want high front down force you can obtain this by the higher front wheel wells with deeper cut behind the wheel well that flows to the upper body. Just like wing concepts the air will flow the same and create down force. However there are more than one way to create a body with high frontal down force.
Steering is obtained by different ways but on the blitz you will notice two rises on each side of the front cut that goes down from the cab to the bumper. Those little wing cuts actually helps steering. The higher sides have created the stability in the cars.
For the average to good driver just using an updated good body and setting the car up to that body is the best idea that was mentioned above. Like the P909 and the R09 mentioned, these bodies have the high sides for the stability and each have a different way to create steering and front down force. So both are good and there are some others as well. I would not say that anyone body is designed for a certain car. Most cars are so similar that they don't know what body is on them.
I will mention that when we designed a body a couple years back we used world class drivers from the USA, China, and Italy. It was so interesting to get their feedback on the different prototypes as we progressed. These drivers were giving me almost the same exact words on how the body made their car react. It is amazing how good some of these drivers are wheeling these cars and how much feel they have. It was a fun experience to say the least. These drivers are the true designers working from the prototypes to the finished product.