Could Mustang have had 2nd stage Super/Turbo without Merlin

MIflyer said:

But the F4F's best speed was at a higher altitude than the A6M.

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'Yes' for the A6M2. But 'not' for the A6M3 (and for the later models).

MIflyer said:

And the two speed and two stage feature meant things could get better at over 15,000 ft, unlike the V-1710 equipped P-39 and P-40.

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True.

MIflyer said:

The FM-2 with a lighter and more powerful engine seems to have been a much deadlier foe than than the original Wildcat - with only a single stage supercharger.

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The R-1820 versions on the FM-2 were with an improved 1-stage S/C vs. the earlier R-1820 models. The power they were making was in the ballpark with the 2-stage supercharged R-1830s in the medium and high altitudes, and a bit better at lower altitudes. Plus, the FM-2 was with the improved layout of the exhaust stacks, basically negating any advantage in power the 2-stage R-1830s possessed in theory. No oil coolers under the wings to add to the drag, one pair of HMGs less, less ammo, a lighter engine - combination that improved both speed and RoC.
Lighter A/C = better maneuverability.

MIflyer said:

The FM-2 with a lighter and more powerful engine seems to have been a much deadlier foe than than the original Wildcat - with only a single stage supercharger.

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Things change with time.
First FM-2 rolled off the Production line in Sept 1943.
In Nov 1943 there are two squadrons doing accelerated service testing.
While Eastern Aircraft built 312 FM-2s in 1943 they had built 1127 FM-1s.
FM-2s don't see combat until late spring or early summer of 1944.

One would hope that a 1943 supercharger was better than a 1939 supercharger.
The engine in the FM-2 made around 80-50hp less at 20,000-30,000ft (margin got closer as the altitude rose.

What was the combat capability (skill of the pilots) of the Japanese in 1944?
The Wildcats were no longer acting as high altitude cover/interceptors for the US Fleet.

The R-1830 pretty much stagnated for several years while P&W concentrated on the R-2800, which was powering much of the US carrier fighters in late 1943/early 1944.
Grumman built 2547 F6Fs in 1943. Over 8 times the number of FM-2s for the year.

This letter brings to mind an incident I read of. A P-40E pilot jumped in his fighter and took off when Japanese bombers appeared over his airfield in the PI. Shoving in his throttle he desperately sought to build up speed and altitude even as the bombs tumbled down. Then he looked at his manifold pressure gauge and his heart sank. He was reading only about 10 inches of pressure! He knew his goose was cooked; clearly his engine had serious problems. But he kept going, trying to get away from danger. Then after a bit he noticed something very odd. The manifold pressure gauge was reading 60 inches, not 10! He finally figured out that the gauge pointer had gone all the way past the end of the dial and kept going, reading well above 60 inches.