are you asking if the Whitcomb rule applies to lower speeds than transonic (600-900mph)in reference to a race car? Is reducing drag your goal or just an equal cross sectional area for adding lumps and bumps?
Scott, here is a very good (and expensive) book on the subject of modern race car aero design. I had it for a day or two and sent it back (cost too much and had a spat of guilt) but it is very informative.
The problem with aero R&D is knowing what the results of your design have done for you in real data. The only way I could come up with to verify improvements was to photograph tuff distribution from a second car and compare to actual lap times, combined with coast down testing. Way beyond my available time and budget.
Please allow me to expand a wee bit outside the initial request Scott. If the mods feel this is extra "noise" and a waste of band width then please delete this post.
The first pic shows how this "area rule " theory was applied to a jet aircraft, while the second explains how the cross-sectional area was attempted to follow a constant curve. Flight -- Live Science Library Ed 2 1981 p.182.
In 1978 Carroll Smith wrote how he got a hypersonic guy from an aerospace concern to go with him to observe a track test session. This guy gave his advise that Carroll needed to see " The man who designed the DC3". Tune to Win p.82.
As I see it the first time someone successfully applied this theory on a racing car was John Barnard circa 1984 on the F1 Mclaren, white car in the middle of these pics.
The pics shows how the other contemporary F1 cars compared with the pinched rear section of the body next to the rear wheels.