[narwan]

Jam:
you're making the false assumption that the vehicles in questions even have a useful AP round. Those gun types affected often have either a fairly weak AP round (not much use at medium and long ranges UNLIKE the HEAT round) or are only equipped with HEAT. Also it is irrelevant whether or not the units in question have a choice in rounds. That changes nothing to the fact that the effect of gun HEAT rounds would be modelled very unrealisticly.

Backis:
the question is not whether we want a flawed representation or none at all, the question is whether this addition results in a more realistic game or whether it does more 'damage' to the realism than good. To illustrate, I've checked just the Iraqi OOB for weapons affected by this change. The following would be rendered useless or close to it with regards to HEAT:
rifle grenades, RPG2, RKG3M, RPG7, 84mmM136, improvised bomb, RPG18, RPG16D, B10 RCL, B11 RCL, SPG9 RCL, up to 3 dozen different gun types (including almost all tank guns of 115mm and less), ATO VFT, flamethrowers, satchel charges, Shmel ATGM, Malyutka ATGM, FFAR's, Molotov's.
That's just the Iraqi OOB. Check the whole game and you can add quite a few more weapons to the list.

I think people are focusing way to much on method here and not on effect. The point of this game is to model EFFECTS, not necessairily actual methods. For example, to give howitzers an AP round without having to give this the same range as the HE round (unavoidable due to code restrictions) these guns got a 'sabot' round. Doesn't mean they actually have sabot rounds, but sabot's have their own range so it works with regards to game effect.
SLAT does not really add 'armor', it reduces the effectiveness of some types of incoming rounds, certain HEAT rounds in particular. This results in less hits on the vehicle. It's a 'percentage armor'; it doesn't give full protection as 'regular armor' but results in a reduced CHANCE of hits for certain warheads. Seems to me the closest thing in the game to represent this is ERA.
That will potentially affect all HEAT warheads but will 'wear out'. Even with the ERA the vehicle can still be hit and damaged and once it's expended all types of HEAT weapons can damage it. Perhaps (and I stress, perhaps) this can be balanced in such a way as to reduce the TOTAL amount of damaging hits on a vehicle similar to the actual reduction (in real life) by the SLAT. The effect will be less succesfull hits on the vehicles without giving them unrealistic 'invulnerability' to lots of weapons.

Just adding to HEAT armor will, due to the fixed code of the game, IMO, do a lot more damage to realism than good.

Narwan