But when measuring efficiency based on changes the power at the pedal only measures output, which is meaningless unless a cost is, as in oxygen usage/hr is used to quantify output as a % of max, if that makes sense. As in a 70% effort with hair vs a70% effort without hair, that is a real world measurable test. This is how running studies are conducted.
The only real way to see if a car saves fuel by having something stripped off it is to run a test with the item attached, measuring fuel consumption and then do it again by taking it off and running the same test, and repeating several times to ensure the data is constant or to find an average.
The thing about theory is it only generally works in theory because us humans mess things up in the real world.
They are testing aerodynamics, effort makes no difference.