The takeaway that I like:

"...so we have proven that exhaust backpressure is all bad, all the time."

Exactamundo, friend.

 

Now I wonder if artifiicially scavenging exhaust via a roots or centrifugal blower may have merit.

Or, imagine, a screw blower made of inconel or whatever that provides constant suction to the exhaust, spinning it fast enough that it provides reduced pressure in exhaust tract everywhere forward of the mufflers. This could yield greater net power, but I do not see how it would yield greater net efficiency.

No one of whom I know has run an engine with exhaust ports joined to a near-deep-space vacuum. An exhaust port that consists only of an exhaust valve, a valve seat, then vacuum would be fascinating, indeed.