Single stage, single speed supercharger on those two Curtiss fighters, along with the Polikarpov, that didn't get a two speed unit till the type 18
A single stage, single speed blower like those were more efficient: used less horsepower to make its boost than a two speed unit did when shifted to 'Low'
But they did run out of boost around 14k, so performance dropped off rapidly. But not as bad as you think, the P-40B took 12 minutes to get to 25k
Now the far heavier P-40D, it took 19.42 minutes, though its time to 10k wasn't that much different than the B model, 3.88 minutes vs 3.35
All fine, but the bolded part.
The two-speed supercharged engine will produce more power in lower altitudes (= supercharger gear in 'low' position) than a single-speed 'highly supercharged' equivalent, it's only gear ('speed') being equal to the 'high' gear of the 2-speed S/C-harged engine. Reason being that S/C will drain less power, since it is turing at lower speed. We know that some engines have had 3-speed superchargers, like Jumo 213E/F, and we could also call the gearing on the 2-
stage R-2800s as having 3-speeds (aux stage being off, then in low gear, then in high gear); the late developments of the Griffon were also tested with 3-speed gearing. Plus, there were DB engines with infinite number of speeds (601/603/605), due to the variable speed of the S/C, and Allison used similar drive for auxiliary stage on it's 2-stage supercharged engines.
There was plenty of low-geared Merlins installed in Fulmars, Seafires and Barracudas - the Merlin III and 45, 50, let alone 46/47 ('fully supercharged') were simply not as good down low as Mk. VIII, 30 or 32 ('medium supercharged'). That is before we talk about the Merlins with 'cropped' supercharger's impeller (45M, 50M).
The P-40D have had greater take off and military power than the P-40/B/C (1150 HP vs. 1040), that helped fighting the increasing weight untlil, indeed, 14k ft.
Now had development of the P-36 continued as a radial than to the Allison and the P-40, it's likely that the two speed Pratt & Whitney R-1830-76 would have been fitted
Indeed.
Though, such a P-36 would've been slower than P-40 at any altitude, especially under 10k ft.