The Germans were using intercoolers on some Jumo 211s.
The 2 stage superchargers on the P&W R-1830s in the Wildcats weren't all that good.
The Two stage Wildcats were good for around 1050hp at around 20,000ft. Accounts vary.
The Single stage single speed Sakae was good for about 950hp at 4200 meters so the F4F looks pretty good.
However the single speed two stage Sakae was good for about 980hp at 6000 meters (19,685ft) and the F4F doesn't look so good.
If they had stuck the Kinsei 51 into a A6M they could have had 1100hp at 6200 meters in 1942.
The Kinsei 62 gets you 1180hp at 5800 meters. A 20% increase in power at just about 20,000ft without needing two stage superchargers.
Yes the Japanese needed to upgrade/change engines much sooner than they did. If they cannot compete with high octane fuel and with high pressure ratio superchargers then use larger engines that are stressed less/ use superchargers with a lower pressure ratio.
And that is something that often is not taken into account. Not everybody's superchargers were even close to the same efficiency.
Granted you have to start from somewhere but if you stick a couple of impellers that are operating at less than 70% efficiency things get strange real quick.
What was not there was engine power above 20000 ft. Lack of hi-octane fuel can be circumvented by a good deal with water-alcohol injection, however there was no circumventing the lack of modern superchargers or turbochargers the West had in use.
The water injection is a crutch.
A lot of rich mixture is actually using fuel as an internal coolant in engines that don't use water injection.
The water-alcohol actually gets rid of more heat per gallon than a gallon of fuel does but there are limits.
RR did a couple of studies or test engines using single stage superchargers using water injection and intercoolers (not in combination on the same engines.)
They were able to get 1150hp from a Merlin at 23,000ft using 9lbs of boost using either water injection or with an intercooler. They were using a higher than normal supercharger gear. Using the supercharger from a Merlin 47 they were able to get 1100hp at 26,000ft using an intercooler (no idea how it was big was) using 9lbs of boost.
The DB605 using the bigger the supercharger from the DB 603 gained around 2000 meters of altitude. It paid for it on the low end. It didn't use an intercooler.
Somebody once said that if your 2nd stage supercharger isn't very good all it does multiply the problems the 1st stage supercharger has.
In the early 30s of lot of companies were using using single stage superchargers weren't very good, but if you are are only using 2-3lbs of boost it is hard to tell.
A lot of 1944-47 superchargers offered much different capabilities than 1938-41 superchargers did.