Gas temperatures per se are relatively meaningless without knowing the radiation charactistics of the gases. Air at 1000 degrees differential has very different characteristics than a luminous flame with the same gas temperature. This is just for fun and contemplation, I'm not sure if it's true or not. That's around 17,600 BTU's. One third of 2 pounds is .66 pounds of wood.
Combustion occurs in a smaller, well insulated combustion chamber.
That adds up pretty close to our original number of 5600.