We put the cooker to a more rigorous test today. Using a different pot, which I painted black, we put 1 liter into the cooker in the late morning. We managed to boil one pot of water wihtin 24.5 minutes, and another one within 34 minutes. In between these two pots we had one pot that did not boil, presumably due to the stronger wind and the lack of a plastic bag.
Our small hand-held temperature probe came down with a heat stroke within 15 minutes of the first boiling test.