Please excuse me, I wish to make a correction, on the 2nd to last sentence, I meant to say “The downforce from the underbody is virtually drag free though”
Whoa, whoa, WWWHHHHHOOOOOOOOOOAAAAAAAAAAA! That's something for nothing. "The second law of thermodynamics declares the impossibility of machines that generate usable energy from the abundant internal energy of nature by processes called perpetual motion of the second kind."
IOW, there is no free lunch. Someone a lot smarter than me figured that out.
Drag from the underbody whether it produces downforce or not already exists on many cars. Subtle changes to the underbody may make significant improvements to downforce without increasing drag beyond the drag that is already there. At no point is this downforce "drag free". The fact that this is possible on a high drag design does not in any way mean the downforce is free. The drag is already there due to poor design.
In fact, changes to the underbody can both reduce separation drag, increase downforce, and with it the induced drag. Then the car makes the same total drag with more downforce. This is still not "free". Drag is still present and a detailed drag analysis can separate the drag due to downforce (induced drag) from the rest of the vehicle drag.
On any vehicle, downforce comes with a drag penalty. This is called the lift to drag ratio or L/D. In ground vehicles we would use the term -L/D. Typical ground racing vehicles that generate significant downforce have -L/D ratios of 0.3 to 3 due to large amounts of separation and low aspect ratios. Air vehicles have L/D from 5 to 20 to 50 for fighters to airliners to sailplanes.
Note the progression: higher aspect ratio wings and smaller fuselages have higher L/D. Cars are the opposite. While we might lump all kinds of "drag" together for a given vehicle, it is more accurate to break it down to each piece and each type of drag produced. Then the induced drag specific to the lift (downforce) can be dealt with against all of the rest.
There is no free lunch.