Ill maybe buy it for the brewing temp bit but not for the steaming.
Steaming has nothing to do with whether water or steam transfers heat to the HX better. Your frothing steam comes from a tap directly into the top end of the boiler, not from the HX. Steaming power has nothing to do with the HX. Discounting the recovery speed of the boiler steaming ability is determined solely by the temp/pressure and the amount of steam available.
Assuming that the temp/pressure in the boiler is the same in both situations you will have better steaming with low water as there is more steam in the boiler.
Java "Isnt Science fun!?" phile