euromodder,
I don't have the technical background to make definitive statements, but it has seemed to me that the "aero-template", for reasons unclear to me, has quite simply ignored half of the equation by presuming that airflow under the car was either irrelevant or beyond the ability of the tester to influence. Its shape is half an airfoil.
Plainly, there are gains to be made by optimizing the under car airflow. Just how large the are I can not even guess, except to infer that they must be sufficiently large that VW and others have made an effort to work in that area on their project cars.
There are some circumstances where an increase in frontal area is worthwhile for aerodynamic reasons. One is
"wheel pants" on fixed landing gear aircraft, although there are additional benefits to wheel pants in that application (mud, stones and other debris are not thrown into the aircraft, prop or whatever). Here are some others to illustrate the idea of making a fairing around the rear wheels.
Chief Aircraft Inc - Accessories - Airframe Accessories - Graphtech Wheel Pants
Wheel pants, ultralight wheel pants, ultralight wheel fairings, ultralight wheel skirts.
Here is a
link to a homebuilt aircraft with wheel pants fabrication shown.
There have even been some fairings on the
front wheels of street rods.
The point being that I agree with you that a fairing over the rear wheels would be worthwhile even at the expense of a small increase in frontal area.
Underbelly pans are complex and have to deal with the reality of suspension components, engine and exhaust temperatures (certainly don't forget the catalytic converter) and the need for access for service.
My thought, without substantiating data, is that attention to the rear underbelly of the car, as you have suggested will likely result in less drag from the low pressure area in "the hole in the air" and associated turbulence that all cars have to one extent or another. Someone or other recently made the pertinent observation that the rear bumper covers of many cars look like a giant scoop to catch air coming out the back of the car.
Here is a brief article about
Richard Whitcomb, father of the "area rule", with some wind tunnel models. His observations are still benefiting the aerospace community and are employed in every jetliner you fly on today, but apply to transonic flight. I think the only earth bound vehicles to employ area ruling are the various supersonic "cars" making speed runs.
Cheers