Frankly I think anything less advanced than the normal Double Heat Sink is too simple to be credible, either realistically or balanced-wise. Even at 5 crits (which is such a light density that it's already disbelievable) such a thing would/should obsolete normal SHS, because the extra heat sinking is too valuable and low tech just has so many crits to work with (no ferro-fibrous, no endo-steel hogging up all the crits) that the increased crit cost isn't really a "real" cost for them.
Maybe it can be made to work if they weighed 1.5 tons per instead of just 1, at a crit cost of 3-4, and also with the extra restrictions that they cannot be part of a SFE's free heat sinks or hidden in the engine, similar to those prototype freezer things.
Primitive XLEs has a similar though not quite extreme problem since for low-tech mechs there's no real issue getting enough crits to fit it. While somewhat more vulnerable than "normal" XLEs it's not that huge of a vulnerability jump compared to them, since a lost torso kills both. It's mostly a way to get more cost-effective XLE lights and is virtually a penalty-free discount for combat vehicles.
Frankly at a certain point, armed support vehicles with more primitive tech ratings become your retro-tech.