So question:
If the guns are modled correctly, why are they not as effective as they seem to have been in real life? I mean if the Ju-87G's really did use relatively shallow angles, the problem shouldn't be one of angles (as most people come in shallow).
If I had to guess, the problem lays in the damage model of vehicles, which require a certian ammount of damage to be destroyed.
Based on observation, it seems that there are two parts to the damage model, those being overall HP and components. In components, there are two separate sub-groups, those being critical and non-critical. Critical components would be engine, driver, and potentially ammunition racks, non-critical components are tracks, and turret. Each of these have a damage threshold that must be reached or exceeded for the component to be damaged. Engine seems to have a second damage threshold which results in destruction of the tank (perhaps through engine fire, or whatever other plausible cause).
It seems (again, based on obesrvation) that to destroy a tank, you must either destroy a critical component (wound the driver, or set the engine on fire), or exceed the overall damage threshold for the tank.
Each shell is also confirmed to have a set starting-damage-value, which appears to decrease with both range (decreased velocity) and armor penetrated (again, decreased velocity). The ability of the round to penetrate armor decreases with range (also due to decreased velocity).
However, the thing that doesn't make sense with this (at least to me) is that several weapons are known to fire APCBC-HE rounds, which have an HE component that explodes after penetration of the armor. IIRC, those are both Panzer IV's (with long 75mm's, I think), Panther, Tiger I and II, and the US M4's. One would think that the explosion in a confined space would do more damage than a shell or fragments of a shell ricocheting around inside the fighting compartment. This would also mean that the rounds from these tanks should do virtually the same ammount of damage regardless of the thickness of armor penetrated (because they blow up after penetration).