It seems to me that the exchanger is not looked at in any way for the heat efficiency stat. All it sees is that you've removed X heat sinks and therefore your efficiency is lowered.

That said, the item itself does work so there are many setups where you are better of losing 3 heat sinks for it regardless of what the bar shows in the mechlab.

I think you just need to do the math yourself :/

True. Items works it is just not abvious in term of graphic interpretation on heat efficiency. Great item for energy weapon boat.

well, the basic one soaks 10% of the heat generated by shots. The sounds a lot better than it actually is in practice.
The things weighs 2 tons, so let's generously assume you ripped out 2 heat sinks.

2 sinks dissipate 6 points of heat per turn, give or take a few percent due to environmental modifiers.
In order to get a benefit over those two sinks, you need to generate over 60 points of heat, at which they break even. Take into account that they do absolutely jack for the heat generated by movement, especially jumping, and remember that the thing does jack besides being expensive and taking up crit slots when you don't shoot.

Also keep in mind that you sink 30 points of heat per round by default, plus another 3 per heatsink, further modified by the environment.

Now, at 100 heat, you will shutdown by default, and take internal damage at about 60%-70% of that on a new character. A total crack pilot can bring that to 130, with no heat damage at all.

But even with those skills: Heat exchangers delay your shutdown at the cost of recovery. Unless you're stuck on martian and lunar maps a lot, an equivalent tonnage of bog-standard heatsink will probably be more useful.

I have no idea if they're of any use over heat banks on gamey builds like "sleepy Awesomes" (basically, an Awesome 8Q with 6 PPCs and no heat sinks, designed to shut down instantly between alpha striking), but as they are they're incredibly niché.

I rather find it an amazingly helpful item.
I have loadouts where a single alpha strike will overheat unless the pilot have 9 in guts for the extra overheat threshold.
Sure on a medium mech it is hard to find a use but on heavy and assault 60 heat is nothing.
My highlander generate 50 heat just from the two AC20's. Swap that for some energy weapons and you suddenly easily break 200 heat on a single alpha.

Extactly that also just do math 10 MLs x 10 heat. That means in 10 salvos you got 100 heat exchanged vs 60 sinked using standard HS's. Easy choise.