By the end of WWII the Germans had M/W systems that could be used continuously for around 30 minutes. Of course that dictates a fairly big tank for the M/W. They needed it because their gas had much lower octane than ours. It is hard to make high quality fuel when your refineries are getting bombed pretty much weekly.
There was a period mid-war, maybe 1943 and 1944 when their planes (engines) were getting their ass kicked because they couldn't run as much boost and they hadn't implemented a way to compensate yet. By wars end, there was parity again, but of course too late. Of course there are lots of other factors, too. The guy who designed the two stage supercharger for the Merlin was knighted, with good reason. The Merlin engine itself wasn't overly impressive in my opinion, but the two stage blower was a work of art. Combine that with good fuel and you can do amazing things.
I don't know if the numbers translate directly, but they had two main grades of fuel, something like 87 and 100 octane. By the end of the war we had something like 120 and even 150 octane, allowing more manifold pressure.
An on top of that we could inject M/W, too. I don't think we used nitrous oxide? But the Germans did.
Aero piston engines are very interesting.
paulie