Eric,
Yes, it's fascination that you can reach very high efficiency with the caldera design.
Insulation of the "reactor" area and oxygen right where its needed is the trick I think, more so than the preheating the air idea.
To further increase efficiency I believe you could insulate more. There is a lot of good ways to insulate, ceramics being very useful. But most methods are heavy.
With thin titanium sheets I guess if would decrease the distance betweend the two sheets around the "reactor" area and have the air standing still in between I guess you would increase thermal insulation so that the loss of preheated air for secondary combustion is a smaller factor than what you gain by increasing temperature in the core.
I've experimented with a thermos, making an air inlet in the side close to the bottom of the cup. First I made it too small, it needed to be a rather big hole to get the right airflow, especially when debris were filling up when I dump various fuel in from the top.
Since there is a whole in the thermos it's not really a thermos anymore, but when the air is heated in the small area between walls I suppose the thermoconductivity is worse than if you had fresh air passing by all the time to be preheated before being let out at the top for secondary combustio.
The thing burned very cery hot, reducing such fuel as acorns, pine cones, dry old food, dry rabbit pellets (yes, poop from a rabbit) anything actually to white ash dust. No visible smoke. No need for secondary combustion when it's hot enough obviously (Heureka!).
I would really like to take a good titanium cup and "destroy" by making a good air inlet close by the bottom. if some of the top is covered, so the heat radiates back while still maintaining good airflow, I believe it would be a great backpacking stove.
An air inlet from below might be even better because of the reduction of loss of heat compared to the heat that radiates out from the hole in the side.
But the hole in the side still has several advantages. It can be directed towards the wind as needed. It's not as easily obstructed by debris and you can poke with a stick if it is. It is dead easy to make a whole. It's within the destructive ability of most of us to make a whole. A nice construction with an air inlet from below demands more skill and patience.
Actuelly you can reach pretty high efficiency wit a regular campfire to, if you have the skill to do it (reflecting stones, reflecting wood, dry spot, adjusted to wind etc). But it takes a lot more fuel and necessitates carrying a saw and an axe, and except for som places (Canada, Sweden, Finland, Siberia etc) there is too many people and too little forest for such a "native" approach. And if more people did it we would probably see an increase of nincompoops who would set fire to a lot of forest. So we'd better cut down on fuel consumtion and have an enclosed portable really small and light fireplace, eh?