I'm arguing against a heat spreader and I've found a small flame that stays within the diameter of the pot's base to be the most efficient. A good close fitting windscreen can also enormously improve efficiency. IMHO a parabolic reflector/heat exchanger below the flame that the pot could sit on might be interesting to try in an effort to capture the heat of the flame that radiates away from the pot. Think of it as the stove's burner sitting at the focus of a reflective and conductive parabolic bowl that supports the pot at an efficient height for the flame. The metal bowl would have insulation on the outside to stop it from re-radiating heat away from the pot and would heat the pot by conduction and also by collimated infra red. Of course the pot/reflector system is a bigger thermal mass that has to get up to temperature, but I think more will be gained by capturing heat that currently escapes than by introducing something else to heat up.