There is more to a baseburner's efficiency than just the length of the hot exhaust pathway. One of the amazing things is how long and how completely the coal burns. This is a product of the firepot design.
Most if not all baseburners have the firepot fully inside the stove body. The firepot is suspended inside the stove body, the outside of the firepot is surrounded by HOT gasses or radiant heat. This means that the firepot is much hotter than the firebricks on a modern box stove's firebox. The result is that even the coal resting right against the wall of the firepot in a baseburner is still plenty hot enough to fully burn, and not leave any BTU's in the ash. The coal is not losing it's heat to the firepot, instead the firepot is supporting the hot environment around the coal bed and this means the coal burns completely
Another feature of the baseburner is the round firepot. While not a rare design feature in many older stoves or in some modern stoves [like the Chubby], it is the most efficient shape for burning coal.
I believe that this is because coal likes to receive heat from all the coal and surfaces around it, and using this heat makes coal burn more completely. The hotter the coal bed, the better the coal burns
The round firepot also usually has a pretty good grate system that lets out the powdery ash that remains after complete combustion of coal.
Many modern square stoves have some issues with burning coal completely, with blocked air passageways, and clogged corners of the fire box. This results in difficulty in shaking down the partially burnt coal, sometimes resulting in jammed grates, or dumped fires.
I'm willing to bet that if a scientific study were done, with any of the modern box stoves, compared to a baseburner or an Oak stove with recirculating back pipe, that the older stoves will prove to be pretty good, probably significantly better in heat output per pound of coal, and completeness of combustion.
There are caveats, an older iron stove must be properly restored and properly sealed, and any and all miss-fitting doors or panels sealed.
So lets say we have a couple of box stoves, and a couple of baseburners and an Oak with double pass back pipe.
Run all the stoves so they have the same surface temperature say, just above the front door of the stove, or a similar location that is similar to all stoves..
Then when running the stove at this given temperature, accurately measure the heat in the flue pipe say, 12" after the stove's breach or exit. I have absolutely NO DOUBT that the flue gas temperature will be MUCH LOWER in the older stoves.
And to carry the experiment and 'contest' a bit further. Put in a measured amount of coal in each appliance, and see how many hours the stove can MAINTAIN the surface temperature in the stove.
Then, measure the weight of the ash from the same given amount of coal, and figure out which stove has the lightest ash, [ most completely burnt coal ] .
Now I'm not a huge advocate of old iron stoves.. personally I don't like iron for stove construction. But if properly designed, maintained, and burnt responsively, it makes a good stove.
I like welded steel for it's strength, sealed welded joints, ease of repair and maintenance. and in some homes, the appearance is better than an old stove. But that is subjective.
I have to say that the big, gaudy, plated monster-sized Art Garland Baseburner I have on display in my home gets positive comments all the time, where a modern box stove probably would be ignored. Liking or disliking the appearance of a stove is purely emotional or subjective. Beauty is in the eye of the beholder.
Your double wall chimney will radiate virtually no heat into the room, the whole reason for the second wall is to create a radiant heat barrier to keep nearby combustibles safe from fire. Only a probe type thermometer into the flue pipe itself is an accurate measurement of flue temps.
Your understanding of the physics of heat transfer, surface radiation of heat, and complete combustion of coal are all lacking. I applaud your enthusiasm for your stove and it's instalation and it's performance for you in your home.. But it is not anywhere near as efficient a design for heat transfer, surface radiation or for low flue temps compared to a multiple-pass baseburner or Oak stove with a double pass back pipe.
Buy a probe thermometer that reads up to 600* or more, they can be purchased off ebay, or used to be. and use this to measure your true flue temps. Then ask a few folks to measure the same in their flue pipes. I think you will be surprised at the results