I did some experiments about 10-12 years ago with heat exchangers. I was looking to maximize heat transfer between a flame (any kind of stove) and the water inside.
Basically, it turns out that by the numbers, there was no difference between using a larger pot (8" diameter) and a smaller pot (4.25" diameter) with a heat exchanger. To the heat transfered into the water, they both looked like an 8" pot for heat transfer purposes.
The smaller the flame, the less efficient the heat transfer becomes, well to a degree, with heat exchangers. Again, it looks like the heat must first warm the exchanger for any usable heat transfer. And, for normal pots, a small flame allows for long heating times. This also allows long radiation times for the heat to escape the pot/water. I could boil water (210f for my purposes due to altitude) but it took about 15-18 minutes. With a blanket around the pot(3/8" foam cozy,) this was cut to 12min and about a 1/3oz of alcohol (.28oz average for 5 runs.) But it sort of ruined the foam. I tried this without a heat exchanger and got nearly the same results about .30oz per boil average. The heat exchanger wasn't doing anything significant. A little more playing around showed that the HE was actually working on excess heat. IE, you need a larger flame to make it work. The fuel penalty was slight, only about .32oz per boil(about a gram difference) for the reduction in time(about 10min,)about 40%.
Anyway, the heat exchanger and cozy only cut the heating time, not overall fuel usage...it was still about a 1/3oz after adjusting to a new burner. I concluded that this was soo close to minimum, that it really didn't matter. 1/2oz (~14-15g) for 2 cups was about as good as it gets in the field, though. For comparison, my WG stove gets about 10g/Liter under the same conditions, by spec (11 liters for 4oz.)
Anyway, it appears that the small stoves without a heat exchanger but with a cozy will max out at around 18 minutes and 1/3oz of fuel. With a heat exchanger and cozy, and a slightly larger burner, this goes down to about 10min on the same 1/3oz of fuel. Efficiency is higher due to time being less, but raw fuel usage remains about the same.
Basically, I believe that heat transfer rates, get maxed at about 1/3oz for two cups. Or, around 10g/500ml of 50C water. From there there are fuel differences, pot size/material/thickness differences, windscreen/cone differences, ambient temp differences, radiative heat losses, laminar flow build ups (probably some I am not thinking of,) that are just as important as a heat exchanger. Heat not only flows into a pot, it will flow out of a pot, simultaneously. But, constructing these things is easy in a lab, but they are difficult to set up in the field. The "fiddle factor" increases along with each additional component...along with overall weight.
A mid-sized grease pot with a built in "fresnell" style heat exchanger to increase the bottom area by about 15-20%, and a tight fitting wind screen is what I use on most camping trips. Either with an alcohol stove or WG stove, depending on the length of the trip. Not the best, but, certainly not the worst. Easy to carry and set up. Easy to use.