UK IFV Warrior, new infomation.

[PlasmaKrab]

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

That is alas untrue. Many coutries are still using and designing advanced light guns.
Look at the modernized Scorpion-90, AMX-13/90, V-150 with gun, or more modern things like SIBMAS-90, ERC-90, Piranha-90...
You have lots of quite modern AFVs with good small-caliber AT guns. Even 76 or 60mm rounds will have a laughable HEAT pen, but quite good APFSDS rounds.

I guess slat armour can be represented by a slightly downsized HEAT uparmouring, say between 20 and 30, to minimize the virtual armour bulk encountered at high angles (however, do as you want, you will still have armour ratings >1000 at very high angles).

With such a value most old or new light-caliber tank rounds can be counted as ripping throught the armour without damage, as Backis pointed out. And I fear such weapons could really be encountered by SLAT-armoured vehicles, even in the near future, since the thing seems to become a standard of the allied kit. Even though might tanks would be surprising, even insurgents could dig out medium AT guns or RCLs.