Hi All.
My $0.02 on this issue.

In regards to limitation of creating more power due to lesser resistance of the air on the prop while in flight.

Your theory about restricted breathing of engines equipped with rotary valve seems to be logical.

BUT, IMO we all should check the rpm of respective engines on the ground and compare them, since most of fliers this days are about brute torque when 3D ing. and not racing the planes around to get them disintegrated by flutter, that makes this argument irrelevant, because we can not achieve any more rpm in hover position than on the ground, by saying, the prop is being ripped somebody mean that they do not use throttle management correctly or prop is to small or wrong load. Most of riping props at our field happens on down lines and not when airplane is pointed up.

I head used ZDZ40RV with DIABOLITON muffler and this engine outperform all others of the same size by far.

And yes, you get 300-400 rpm with tunned canister and you can use it when you need it, on up line, on the other hand your engine does not run out when you forget to bring the throttle down on the down line. In the way that helps the pilot .

Unless you are racing pilot, you do not need more rpm (power) with increased speed, that is my logic.

There is about 6 ZDZ at our field in use and all of them are superior in power to anything else of the same size. that would proof's the point that rotary valve is superior for our application.

I have never seen another 40, 60, or 80 developing same or better power than ZDZ.

Just to let you know, I am not bias, I fly Brison, 3W, ZDZ, BME, WEBRA, about 15 in all.

Thanks for reading, Jack.