I'm guessing that the break-even point on space efficiency is somewhere close to the point where you invent Blade Runner-style levitating cars that require zero

I'm guessing that the break-even point on space efficiency is somewhere close to the point where you invent Blade Runner-style levitating cars that require zero extra space for take off and landing, and can move arbitrarily slowly, or even come to a complete stop, without falling out of the sky. Without that, I doubt that that adding some room for vertical stacking would offset the amount of additional lateral space they end up needing. And I believe that effect would be intensified by the the amount of vertical stacking you can reasonably do being constrained by how far a vehicle needs to move laterally in order to gain or shed a given amount of altitude, and the distance of the trip being made. For a traditional fixed-wing aircraft operating over "urban commute" distances, that constraint would be downright tyrannical. For Blade Runner style levitating cars, it would be no big deal.