Just an FYI: the 37mm's from the Yak9T and P39D/Q fire AP rounds. All four 37mm's in the game (M8 Greyhound, Il-2, Yak9T, and P39D/Q do the same 17.1 damage to soft objects (ord, fuel, barracks, radar, hangers, etc). If anyt of them were HE rounds it could be logically assumed that the damage would be higher.
I don't think that's true at all. You're ignoring the fact that damage vs ground objects is nowhere near the level of detail as damage vs planes and other vehicles. This was noted even many years back when the "rounds to kill a hangar" was first introduced as a testing measure.
What you claim is tantamount to saying all the 20mm in the game are the same. Given the evidence so far, HTC doesn't work that way.
EDIT: This page:
http://trainers.hitechcreations.com/guns/guns.htmWhile outdated (not including P-39 guns or IL2's AP loadout) blows your argument out of the water, showing widely different values for the ostwing 37mm HE, the NS37, and the M8's 37mm gun.
EDIT2: heavy caliber AP rounds against fighter planes was not the norm. The heavy round's strength is the explosive content, not the punch-a-bloody-big-hole content. AP would be a waste of a round against another plane in the 37mm size.