A Mustang, being faster than a P-40, can generate more RAM. If the supercharger can supply 65-68in static at sea level perhaps 340mph+ can supply another 4-7in of pressure?
from a test the British did a Mustang with a -39 spec engine (later engine refitted with 8.80:1 gears) held 56in to 7900ft or a gain of 3600ft in altitude of "static" or looked at another way around 3.2 in pressure? Perhaps less after before being multiplied in the blower?
While these are very impressive numbers at sea level and help explain the combat performance at low level that seems out of line with the "normal" book figures (Military power rating) it is also obvious that the power goes away very quickly with height and is even worse when trying to climb.
http://www.wwiiaircraftperformance.org/mustang/ap222speed.gif
If you follow the curve down and extend it the engine might have been able to pull 60 in at a bit over 6000ft, but a P-40 would be at a lower altitude because of the lower speed.
or this test with a -73 engine?
Mustang II Performance Trial
Held 60in to 4400ft instead of the chart 2500ft for static but look at the climb
Held 60 in to 800ft and was at 55in at 4000ft. In level flight it could make 50in at 10,000ft but in a climb only 45 in. Roughly a 10% loss in power in the climb vs full speed level flight.
Again, they were
not over revving in the tests.
I don't doubt that those boost pressures were used or seen at times but unless the plane is right at sea level the ability of the plane to use them is limited or non-existent without over revving the engine and even a few thousand feet of altitude can make a big difference in power, especially trying to climb. The test Mustang lost 20% of its climb rate by the time it hit 6,000ft.
The same engine, installed in a P-40 or P-39 (same spec) will give different full throttle heights because of the different air intakes and the different amounts of ram.