XP-40Q-2: test data emerged!

Neither plane had armor behind the pilots head. What they had were turn over pylons to help prevent pilots form head damage/broken necks incase the plane nosed over on landing.
I have no idea how easy it was to see past these structures and out the window panels in the rear and yes the Severski prototype does have a rather wide strip of metal at the top.

tomo pauk said:

You can have both high altitude benefits and WEP without intercooler, at least for the 2-stage V-1710s.

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Yes, but the WEP is less than it would be with intercooling or ADI.

At any given altitude, intercoolers and ADI will allow more boost.

tomo pauk said:

Water injection can certainly be used during the climb. Having an intercooler many times meant increased drag, so not having intercooler also helps with high speed cruise.

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Yes, ADI can be used in climb, but with a limited amount of ADi mixture available, any used in climb cannot be used in combat. Or you carry more (= more weight).

Yes, an intercooler/aftercooler will increase drag, but it also increases the efficiency of the engine.

tomo pauk said:

Is there a breakdown of monthly production for the 2-stage V-1710s available?

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Best guess about 200/mo. 5013 two stage E models for the P-63 for 25 months (April 1943 through May 1945). P-63 cancelled right after VE day. Not including a few F models that didn't get into production.