The majority of the heat transfer is in the top. A square or flat surface is the best transfer. This shape gives you larger flat surface for this transfer. The round simply does not transfer the same it funnels the heat towards the top center produces a very hot center but the majority of the transfer if funneled to the very top. The best transfer is a square but provides a horrible self feeding environment, you end up with a burned out middle and wood left on the sides. Round provides the best self feeding environment but not as good heat transfer. So if you combine the 2 you have the possibility of a few shapes, we like the octagon but other shapes will work also.
We have built various types and shapes this is the fifth generation boiler, I personally have built about every shape I could come up with and we will see additional improvements over time. But I can say that all our burn chambers will continue to be octagon.
Thanks, Blaine