I love these discussions/debates.
First, investigate how the damage dealt by the different ordnance. Bombs, rockets, and HE shells (and AP) are all scaled, to a degree, based on the objects (OBJ) available to destroy. I can post the damages if need be.
Likewise, each of the OBJ have a hardness setting. HTC has scaled, it appears, the different OBJ based on the amount of damage they believe needed to allow the widest range of aircraft and gv's able to get the job done. Ever notice that it take two rounds of HE from a King Tiger all the way down to the lowly Panzer IV F/1 to bring down a typical OBJ (ammo bunker, radar tower, fuel tank, barrack, town building)? Mind you, the M4/76mm, M18, and Firefly all need 3 rounds due to their lackluster HE round, kudos to HTC for modeling that correctly.

Yet, the King Tiger, Tiger, and T34/85 all do 234 lbs of damage to the typical 156 lbs of damage (Panzer IV, Panther, T34/76, etc). The M4/75 does 178 lbs of damage. Point being, HTC has tiered the damage system to try and keep things "level", to a point. Now when going up against a hanger or other such OBJ (shore batteries, HQ, etc), the M4/75mm is supreme thanks to its 178lbs of damage and 4 second reload rate. The Tiger is next in line, then T34/85, then King Tiger, respectively due to their reload rates. So the heavier calibers are only really given their due against the larger OBJ.
I believe HTC should almost start from scratch in terms of OBJ hardness settings. Each OBJ should get a different hardness setting based on how fragile it would really be. A reinforced concrete bunker should NOT have to worry about most guns, cep't maybe the IL-2, Hurricane IID, and Stuka G-2.