There is a lot of behind the scenes stuff that often does not come out in many books. I don't know what the actual status of some of the German progress was but some of the American progress may or may not be helpful.
The P W R-1830 started at around 2400rpm and went to 2550/2600rpm and then to 2700 rpm ( radials are harder in increase rpm on because of 7/9 cylinders on one crankpin). At some point ( I forget which) they changed the material (alloy) of the plain bearings they were using and this allowed the higher power/longer life and was judged to be such a success that P&W used on not only their own engines but shared the knowledge
without compensation with air aircraft engine makers.
Airplane engines are not like car engines, it is not so much a matter of "tuning" for a higher rpm as making sure the bottom end holds together at the higher rpm/power. There used to be a semi joke about racing old Triumph sports cars ( the big 4 cylinder models) that said if you could turn 6000 rpm you were NOT making competitive horsepower. If you were making competitive horsepower the bottom end would fail at 6000 rpm.
Airplane engines ( at least the supercharged ones) walked a much finer line between weight (strength) and power. The Allison for instance used three different crankshafts that all weighed the same. The early ones were plain steel, followed by the
same crankshaft
except shot peened. The shot peened crankshaft would stand over 20% more stress for 100,000 cycles and would stand up to 95% of the stress for millions of cycles that would cause the plain crank to fail at 100,000 cycles. The SHot peened crank was followed by a nitrided and shot peened crank that showed an even bigger improvement. The nitrided crankshaft (introduced in early 1942) would run forever ( well over 10 million cycles) at a load 29% higher than the plain steel one would tolerate for 100,000 cycles. Later on Allison introduced the heavier counter weighted crank for even more improvement. Trying to use late 1942 or 1943 WEP power levels on a 1940 engine with a plain steel crankshaft just because you now had better fuel means the engine isn't going to last very long even if it doesn't suffer from detonation.
Maybe the Germans were already shotpeening and nitriding, I don't know. But if they were they might have been using the better strength to keep weight down rather than leaving "room" for later increases in rpm/power.
Better aluminium casting techniques allowed Allison E/F crankcases to require 10% less machining, be lighter and yet stronger than the "C" series crankcases.
Most of the time descriptions of engines focus on rpm, compression ratio, blower pressures and so on. Things that can easily be measured. Changes in materials, foundry techniques, alloys and heat treatment that
allowed the engines to stand up to the higher power get much less attention. Like wright changing from the aluminium crankcases to steel ones to get 100 more rpm on some of their engines ( they also operated at higher boost/cylinder pressures) in some cases the crankcases were worth about 100 hp on the R-2600 engine. They didn't
make the extra power but they
allowed the extra power to be made with the same or longer engine life.
Just changing rpm by few hundred could cause a host of vibration problems because most of these engines used the lightest crankshaft they could, bare minimum of counter weights for the designed rpm.
Another photo of a Jumo supercharger impeller can be found here :
1943 | 2104 | Flight Archive
Somebody has to kick Junkers in the tail to stop them from using that impeller well before 1942/43.
You are correct that the Germans might have been better served by developing the existing engines rather than branching out to some of the odd balls.
But something to remember is that the odd balls were built to get
around certain well known problems Like piston speed and cylinder size limits and
not just because the designers thought they were clever.
Another thing to remember is that many of the aircraft engines were operating, on a power to weight ratio, on a comparable scale to some Grand Prix racing engines of the time.
Except the aircraft engines were running on gasoline instead of exotic racing fuels and the aircraft engines had much greater durability. In other words the aircraft engines were every bit as 'cutting' edge as many Grand Prix engines and there was nowhere to go for help.