I guess I said things a bit wrongly -
The belly drag and pressure comment referred mostly to stuff far dirtier than the present line of disposable cars.
I live near Bonneville Salt Flats and my passion was racing for many years. (starting in 1954).
Cars like my full size Chev van really have dirty bellies.
A belly tanker has a clean (but salty) belly ----
The spoilers I scoffed at are the 200 mph stuff on a 100mph sedan.
Is is amazing tho -at how clean come of the new stuff really is -and the lack of wind noise in even the lower priced offerings is really notable.
I remember the KAMM back stuff from earlier times - which sure turned out to be an effective (tho homely ) fix.
Having sold cars - I know first hand that you can get a lot more milage out of showing off the kooool spoiler than talking about flat floorpans etc.