the amount of energy lost bouncing vertically is probably adequately absorbed by the tire, fork, and rider such that it's still going to be a faster tire.

Yes, Fun conversation. Thank you for continuing it.
I will stand by my theory that the energy the fork, frame and rider absorb (convert) is wasted. Yes, you can run too soft... but with an appropriately inflated tire, energy taken up by the front of the contact patch and returned at the aft is not wasted....thus a faster more efficient set up.
It would be interesting to do the same road with a powermeter and a speedometer. How much power does it take to hold X mph with a range of tire pressures?
We need a tire engineer to jump in here.