But (with exerience) the 20mm (with it's high rate of fire esp.) can ruin a tanks capabilities in a minute.
I wasn't there but I trained on the Leopard 2 A1 (nearly the same as the M1 Abrahams MBT) and I saw the Marder A3 (APC) in action.
A trained gunner can disable the following things on the Leopard 2 :
Main Gun optics (very easy, either with MG or the 20mm MK), the commanders periskope (a bit harder, best from higher grounds with HE rounds). The computer (for range/stab etc) at the right/back corner of the tank. Various viewing slots (though they are a periskope and you wont hit anyone inside through them there are only 2 spare ones on board). The antenneas at the backsides of the turret (quit hard to destroy, but a few HE at the base will do). The AA-MG at the loaders hatch. And any equipment outside (doesn't matter). Severly damage the engine (exhaust outlet/air intake) from rear/higher firing grounds).
So even if the Marder with it's small 20mm will not be able to breach the Leopards armor from any side it's able to nearly completely disable it. Esp. with the weak training of operating the gun without computer/range finding the tank wont hit anything

So my question is : Shouldn't the damage chance on tanks be adjusted by the experience ? Probably this will be hard. Esp as one cant 'choose' which ammo a tank should fire. Together with the way damage on tanks is calculated I guess the game mechanics wont give a easy way to resolve this.
But still it would be nice

murx