The surf hull is relative narrow so the transition for “getting over the hump”, in to planning mode is probably not that visible on a surfboard as any other planning hull so your observations are accurate.
3.6KW on prop shaft should be 30 +km/h, but we have pretty much power losses on the way from the battery so that explains the difference. 1.2kw for foiling same speed is indeed a huge difference, comparison is hard if hardware, board an possible weight is not identical. Anyway still significant gain.
My wild guess is that on higher speeds the fixed size foil will loose advantage(in calm waters) as the vetted surface will remain the same but on no foil it will decrease.
I will start my project without the foil, lets see what the result will be. Keyboard building is easier than actual building:). Now have some parts, 2 esc but only one motor, one lost in the mail. Aliexpress is OK only if you have very good patience…