Rindert that does look super efficient during heating, to run the combustion gases back down over the outside of the tank. For storage I don’t know how that would work, I think a lot of heat would get lost during “storage” time, i.e. when the water was up to temp, and no hot water was being demanded. With the tried and true domestic water heater design, there isn’t a lot of heat escaping out the flue that needs to be captured, and you do need a little heat anyways to keep the fire drafting nice.
However, making steam is a whole different ball game. Seeing the term “superheat” gives me the shivers. All we’re doing is heating water to a little over a hundred, or maybe 150 if someone got a bit zealous with firing. The safety valve opens at 160 which is still well below boiling point at atmospheric.