It seems like the ideal case is to get the flame on the bottom of the pot only, not coming up the sides. This has made people look to get wide bottom pots. However I've heard one voice saying that he tested this and found no appreciable difference in efficiency so he stays with a narrow pot which is the lightest option that he has.
It makes sense that if the flame comes up over the side there may be a loss of efficiency, especially if the flames come over the liquid line in the pot, but really I took that at face value and didn't question it. So was wondering if anyone had any data points on this from actual testing. I may do some and report back.