Like you said, this is empirical but fortunately, Thermodynamics is on your side with a heat shield! The heat shield will act as a heat director, sending heat up at your kettle so that the heat hits the bottom of the kettle, then pushes out around the kettle. The way to look at it is simple: heat on the side of the keg=not so effective, heat under the kettle=effective. If you look at the jet boil http://www.jetboil.com/ they keep the heat in with a heat shield and a heat transfer fin. I would definitely make some heat shields for your burners. You can make them out of sheet metal cut to size, then bent into a circle and screwed together or bound together some other way. I think there is a thread on here about this. Making shields this way will cost you at most $20 for three burners as opposed to spending $100+ on shiny stainless ones that do the same thing.