After browsing through real world and AH data, as well as conducting a few tests offline in different tanks, at different ranges & angles I came to the conclusion that this is most probably not a topic for the "bug report" forum but for the wishlist.
While with T-34's we have loadouts that include only a few of the rare HVAP rounds, tanks like the M4, the Panzer IV and the Panther are fully equipped with a full loadout of the best, but also most rarest ammunition. In reality the PzGr 40 was very rare to be had in a Panzer 4 (the supply of tungsten basically ended after 43) and mostly issued to tank destroyers, if at all. In AH it is the only available, standard AP round. Same seems to be true for the M4 and the Panther.
Until we get a perk ords system that allows you to "buy" rare high power ammo, I'd suggets that we get loadouts for those tanks that do follow the T-34's patter, i.e. only a few APCR or APCBC rounds per option.