It�s hard to rate the increase in heat output because there are so many factors to consider.
First it has a baffle plate and it's own A-framed spark screens which not only smashes sparks but reflects heat back into the fire box. A hotter fire box with cooler pipe. A stock stove system can get hotter than one with an heat exchanger for a few minutes as the whole pipe may turn red during the height of a good burn. When using a heat exchanger I can often touch the pipe 3 feet above the fire box for a short time even with the robber cherry red. But the longer hot more even burn puts far more heat into the tent than a more extreme shorter duration spike. Also aids in more complete combustion aka fewer coals in the fire box to restrict air flow and clog the smaller fire boxes of these backpackable woodstoves.
Second there seems to be a secondary combustion of any un burnt gases within the smoke so it dumps more BTUs into the shelter not out the pipe. This combustion seems to compensate for the reduced airflow which could be why I can run it with up to 3 spark arrestors, two inside the Kifaru collar and a Ti goat inline I got with the cylinder stove plus an adjustable damper to boot. This is no easy task in a take down stove though it works best after the first few stokes.
Third it allows more room for the flames to expand into. Kinda like an add on fire box that can be removed when not need.
The net result is a longer more even burn as the robber turns red for the majority of the burn. In the case of the my homemade more (relative to the Kifaru) airtight stove this burn has been clocked at the ECR for around 55 minutes of real heat output. I timed the less air tight Small Kifaru stove at 35+ minutes. This is a substantial increase over the usual 20-25ish. It makes a BIG difference in ease of operation.
A robber isn�t all that easy to make right as you need to find the sweet spot in relation to volume, air flow and weight. I gotta make a non welded take down UL version this summer. Somehow think a sideways mini cylinder stove might be a good way to do this but still unsure. Maybe I will just do up another SS container but somehow have the inside accessible for gear storage etc. Again not sure.