[narwan]

Jam:
Again, many of the lighter and older guns have no AP (or sabot) rounds. The bmp-1 's main gun will have little effect for example.
I don't think flamethrowers, FAE weapons, explosives packs, improvised bombs and gun rounds will be stopped by SLAT and these are all used as AT weapons.

It's also not just a question of whether other rounds will still be able to penetrate, but also one of range. AP rounds of the older and lighter guns will have not much actual penetration left at the longer ranges which are now common for armoured combat. If they hit a vehicle at an angle in particular they often won't be able to penetrate with AP rounds where they would have been able to take out the vehicle had they used a HEAT round.
Newer rounds will also have problems, the higher the HEAT armor, the less chance of actually penetrating. Having a HEAT penetration rating higher than the armor value does not garantuee penetration.

There's also the question of whether and how it will work in combat conditions. Others have mentioned that it won't be able to stop all of the rounds it is supposed to protect against (RPG's for example). That means some will destroy these vehicles. So from that realism point of view if you model it via heat armor values the armor value should be less but close to the penetration values of these weapons. Or you should greatly increase the chance of hitting a weak spot for these vehicles to mimic some of these rpg rounds getting through. But that would make them more vulnerable to other AT weapons too.

There are indeed significant differences between SLAT and ERA, but just adding to HEAT armor doesn't do justice to realisme either. Using 'ERA' values for the game might come closer to reality, maybe not. But I think it's at least an option to consider IF you'd want to model SLAT as it may be the less unrealistic option within the available game mechanics.
Short of a new armor value specific for SLAT it may not be able to effectively put it in the game without these unwanted side effects. It wouldn't be the first thing which doesn't fit within the limits of the game. There is for example just 1 regular and HEAT value per armor side while the actual thickness often varies on such a side. The lower front hull of tanks is usually less well armored for example than the upper front hull.
While I would like to see as many options and new inventions included in the game, there isn't room for everything unfortunately.

Last point, and a new one; will the addition of the SLAT have an effect on other game stats? I'm thinking speed in particular. There is the additional weight which might reduce speed, or possibly other payloads (troops, ammo, etc) if one wants to keep below a weight limit.
Also the increased dimensions could make the vehicle more unwieldly and more difficult to manouvre. That too is represented in the speed rating. Should this (or other values) be modified too? Just asking, 'cause I don't know.