Thanks for your interest.
Yes the Glenwood does make it obvious that a lot of thought went into the design in little things that might not be so apparent in other stoves. Even the footprint is not square as it is in most oak stoves. Making it a little deeper allows the ash pan to go deeper to catch that ash at the back.
I can accept your statement that the large area above the fire was thought necessary for good combustion but it does seem to me so large as to not direct flue gas closer to the walls to enhance heat exchange. This is in the simple oak stoves and does not apply to those with extended flue passages such as yours. It might be that the makers wanted to keep the stoves as simple as possible to guard against the usual neglect of many users and their failure to clean more complicated configurations. Price also is a consideration.
I have been running the stove with manual damper only. Output is very sensitive to the position of that damper. With both air shutters open about 1/16 inch and damper closed 3/4 results in a stove temp. of about 400 with a stack of 230 or so. Leaving the damper open and adjusting with one air shutter alone open about 1/16 inch results in the same stove temp. but stack is about 30 degrees lower. I do believe it is a more efficient fire and I am getting a more healthy quantity of the other blue flame that only appears in a fire long after initial volatiles are burned off. This has to be better burning of CO caused, I think by, not more air, but by higher velocity air mixing better. This is very subjective and I will have to study more. Even with such a small quantity of air the round chamber is still maintaining a circular pattern as you can see by the lean of the flames, small as they are.