Actually, Hermit, a lot of the answers to your query CAN be found by looking at the after-market components made by GM Motorsports and used by the World Challenge drivers.
On 2001-04-16 23:29, Hermit spewed forth this drivel:
Regarding the aerodynamic aspect I would question whether any of the aftermarket parts could demonstrate much improvement in air flow. Considering the state of the art wind tunnel and computer testing that Chevrolet does with these cars the chances that a small business without access to some pretty sophisticated equipment could improve on their design, especially when you consider the requirements for all around, real world performance, is pretty slim.
A low wind tunnel number is only one piece of the puzzle. Just because you have a low drag number doesnt' mean the car will handle well. In fact, over 90mph, the C5 can become a little squirrely in the back.
I will grant you that small companies can't access high dollar wind tunnels and such. But, it's not hard to figure out how to make an airplane wing. /phpBB/images/smiles/icon_smile.gif That's all we're talking about if you simply want a low drag number.
If you look at what wind tunnels are used for by today's race teams, it's how to move the air around the car and use it for better handling. In other words, how to "balance" the car.
A small lip on the rear fascia of a C5 WILL stabalize the car. This is a common fact that has been well established and doesn't require a wind tunnel. As Bob Dillon said, "you don't need a weatherman to tell which way the wind blows."
The trick to the spoiler is using it to control the balance of the car for better handling. This is a race tested/proven process. Not a wind tunnel issue.