It does seem possible that the downside of the OP (Opposed Piston) engine may be it's breathing. As it runs faster it has less time to clear the spent gasses from the cylinder and can only do so through the ports with pressure from the intake system. This may mean a large parasitic loss to run the blower fast enough and at high enough pressure. and it may mean a gradually more and more contaminated mixture at higher and higher speeds.
Detroit Diesel two stroke engines are a similar design with a roots blower and scavenging ports. But just one piston per cylinder and with cylinder heads and valves. They respond well to turbocharging.
As far as the horsepower required to run the blower, and just as an interesting side note, I had a friend that had a dragster. It was powered by an older generation Chrysler Hemi with a blower. I remember him telling me the blower took 400 horsepower to run it! That was an extreme case, but interesting, I thought. In spite of that parasitic load, the engine was still producing around 3,000 horsepower!
I'm away now, but when I get home I'd like to study these things further and look at their history and the charts that Achates has come up with to see some trends in their behavior. This should speak to scaling the engines.
OP engines work wonderfully as slow ship engines, but light weight
and high speed ones may present some serious limits. Dunno.
I was a throttleman in the Coast Guard for a while and ran two of these out at sea. In that application, the engines were stopped and then started in the other direction for reverse. They were started by injecting air into the cylinders through starting ports that were fed by an air distributor system that looked a lot like an ignition distributor on a conventional gas car engine. The timing of that distributor determined if the engine started in forward or reverse. As part of routine maintenance, we would take a reading on the rod bearing clearances. This was done by removing the fuel injector that pointed in right between the pistons at top dead center. The injector was removed from each cylinder and we slipped in a 1/4" diameter piece of round lead rod. Then the engine was rolled over to smash the rod between the pistons. Then we would use a micrometer to measure it's thickness and use that number to determine if the bearings were OK. One of the engines developed a grinding sound near the upper crankshaft and the vertical shaft interface, so we pulled the upper crank out at sea (!!) to have a look. That is when all hell broke loose and we got into an Atlantic storm. It's quite a story that I won't go into now.