I doubt that a caldera cone would be much more fuel efficient with a jetboil flux ring pot. I'm sure they could make a caldera no probs for it, but I doubt youll see much extra efficiency.
The flux ring increases surface area of the pot, which increases heat transfer to the water. But, using the CC kind of does the same, by trapping heat onto the sides of the pot for much longer.
I dont have a CC, but I've noticed that very little heat transfers to the side of the jetboil, via the outside at least. What does seems to be emanating, is more likely from the water on the inside through the neoprene.
This would make the CC kind of redundant, except of course for its stability and ability to block wind (which is the downfall of the Jetboil-making it about the same efficiency as a normal canister stove).
I figure that if the caldera emanates little heat from the outside of the cone near the top, then most of it is probably transfering to the water in the pot. Someone with a CC can probably tell us if this is happening. This would in turn make the jetboild pot pretty redundant.
With only slight improvements potentially possible on fuel consumption, the considerable extra weight of the Jetboil pot wouldnt really be worth it, until someone makes a much lighter Ti version. BMW?
Another consideration is that a CC probably won't get fast enough airflow for using a canister stove-not without significant extra air intake holes, so it wouldnt burn as efficiently as it would in open air.
In terms of making the CC more efficient, I am thinking more along the lines of adding a thin layer of neoprene to the outside of the cone near the top....?
Sorry, this was an essay that may have covered some points made already.