Craig: The drag/downforce rule always applies, but in this specific application we actually
lower the drag
a lot by adding a flat floor. That alone will give you a big slash in laptimes.
So in this case it would be more like smoothing the body then to actually add a piece to aid downforce. Downforce is a nice
free lunch when smoothing the floor so to say.
Z: I totally agree, just as long as you know why you do it it's fine.
Anyone can gain access to a wind tunnel, with enough cash.
A scale model replicated in good enough detail to work as a real world test bed would be too expensive. Also, using a wind tunnel to simulate the floor aero is very difficult because the car is not actually moving realtive to the floor, there are ways to simulate that with rolling belts for instance but it will always be a bit different then in the real world (add to that the stuff that Craig mentioned about the car moving about in pitch/roll/heave). Did I mention it would be expensive?
For pressure distribution along a section of the body you can use any general section from a sedan available, they don't differ that much.
pressure measurements are indeed cheap and interesting, they do not tell the whole story however. But they are a nice tool.