But I think your experiment suffers from inaccurate assumptions. Coming from a scientific background, I can attest that real world experiments are extremely sensitive to initial conditions and you must be very careful in how you set up the experiment.
Double wall woodstoves are based on the "reverse downdraft woodgas" stove principle, which is a modification of a forced air downdraft stove. (obviously, reverse downdraft means updraft or natural convection) That means that the fire starts at the top and migrates downward through the fuel. In this mode the flame front stays small and is close to the woodgas being produced, thereby creating more heat but less upflow of air. (the woodgas is actually being produced from the radiant heat of the flame in the fuel BELOW the flame, which then passes up THROUGH the flame to be ignited) The insulation of a double wall stove helps hold the smaller amount of heat in, and preheats the secondary air right at the point of combustion.
When you light the fire from the bottom, you completely change the physics of the burn. The bottom fire heats up all the fuel above at one time, creating the huge cloud of unburned smoke, yet the fire itself is inches away (at the bottom) from the woodgas produced. Therefore, you waste a huge amount of the woodgas before it ever ignites. Once the flames propogate up enough to ignite the woodgas, you get a huge burst of flame and burn through your fuel very quickly.
Since there's this big fireball, air is sucked through the stove very rapidly, which pretty much negates any insulation quality of the stove itself. The stove just becomes a container for an open fire, much like an HVAC vent just moves hot air from one place to another without picking up much of the heat.
The key to an efficient and hot woodgas stove is to keep the burning woodgas inside the stove (and under the pot) for the maximum length of time to allow all the gases to burn. When your flames lick up around the outside of the pot, you're just heating the outside air, not the pot itself.
What you want is a slow, tightly contained flame that burns smoke free UNDER the pot. A double wall stove will help in that regard by preheating the secondary air. (The primary air supplies the flame front that propogates down, the secondary air mixes with the woodgas above the fire to insure complete combustion of the woodgas)
I would suggest running your experiment again, but ignite the fire from the TOP. If you use fluid to start the fire, make sure it doesn't drip down into the wood at the bottom and end up starting a fire there. Best way to do that is with a cotton ball soaked with a few drops of kerosene placed on top of the main fuel with a little kindling. (wood chips)
If you enclose the area between the stove and the pot with an enclosed standoff, you will also increase the secondary draft while heating up the woodgas and air mixture for more efficient burning. If your flames are licking way up the sides of your pot, you've got too much primary air or too much fuel being wasted.
I think under these circumstances, you'll find the stove to be more efficient and the double wall and insulated modes to heat water faster, with less smoke.