I've said it before and I'll say it again, USF should have had the
3" AT gun, with 57mm being reserved for paratrooper doctrine. It would have performance more similar to Zis-3 and Pak40.
The other thing would be that IMO HVAP ammunition should be a global upgrade, not a vet ability. Like the T4 building or something would have "HVAP requesition" upgrade, which unlocks HVAP ability on all relevant vehicles and guns. It still costs munitions to use, but you would at least have late-game access to this ability on your fresh units.