the current hangar tonnage heavily favors US planes for fields attacks, reducing the diversity of the planes in the air...
While I see your perspective on that, I think that the reason why we don't (and won't) see p40s or any other low ord loadout plane, is because most of them simply don't perform as well as the late war monsters.
P38s and p51s are planes I see the most with ord attacking anything. Also, Tu-2s don't carry many ords, yet lately, I have run into more TU-2s that I have any other bomber. I have only seen 2 formations of lancs and 1 of b17s. I'm not sure that lowering the hangar health would do anything good. While I do support it, I am not convinced that it wouldn't be abused in some way.
It simply gives even more power to the late war monsters. Assuming it was added, and the Hangar health was reduced.. sure you can 'now' kill hangars in some of the mid-war planes. However, late war planes have more of an impact. Sure, you can perk them, but that doesn't mean that it will stop the flow. B29s are perked, and perked high, yet they are still used and have massive damage potential.
Again, I am not against this but I am not convinced that it would go as planned for the OP and all supporters. If it was to be added, it would need some extremely delicate balancing to adjust to all the ramifications aforementioned.