Dave,
"swirl recovery" and self canceling rotorwash, as mentioned by superiorheli above.
Can you elaborate on that?
I would think the downwash speed of the Kmax would be similar to other helos of similar disc load (maybe higher), is that not correct? The Kmax is efficient because of the large slow rotor more than or in spite of the intermeshing configuration. Maybe the customers (loggers) just didn't want to deal with tail rotor strikes.
Two coaxial props on an airplane regain some from swirl recovery, but still not as efficient as one large prop, I think. Coaxial props are used on very fast or over powered airplanes. Not used for efficient personal planes. I have not ever seen a coaxial or intermesher of full size or model fly.
But I am still interested.