I think further fracturing of classes in the SCCA (sports car STL v. non-sports car STL or whatever) is a huge mistake.

That said, I understand the issue and that I am part of the problem. I probably WOULD build a FrankenNSX or an Esprit or some nonsense for the class....

It seems to me if you are at the "how" v. "why" stage that the real difference between sports cars and non-sports cars is frontal area. If so, what you are looking for is a CD X frontal area modifier of some sort.

Beyond that, I think any of the characteristics you are looking at are red herrings.