how late did the improved HE shells come out? If its like Febuary of 1945, then I don't think they should be represented in game.
However, I do think that the German and US tanks should also recieve a few rounds of APCR ammunition as an option. Say 5 per tank, since although they were historically used, they didn't see large use in the tank units.
Germans just never produced large quantities of the rounds IIRC, and for the USA, AT artillery got first go at the shells, and the TD units got second pick, and the tank units got what was left over.
Say, maybe 10 rounds of APCR max for the M18, to represent the greater use with tank destroyers?
For the brits, give them a similar number of APDS rounds, but have them do even less damage than APCR rounds do (smaller projectile).
Or for vehicles that used APDS more widely, such as the Cromwell and Valentine (with the 6lbers), we could give them a larger loadout of APDS as an option.
Or, we could just add on an extra loadout option including APCR/APDS rounds, that we can disable by arena if the rounds weren't used during the time period.