Yes you are right you could approach that from various angles, but I simply mean going as fast as you can in the least possible amount of wind. Lets call it the boat speed/wind speed ratio. MI is at about 2.5:1 , kite record is about 1.2:1.
It's just too hard to know, from where I sit anyway, which record is purely more efficient in terms of 'energy in - energy out'. If you look at 'sail area' the ratio is about 5:1, going on your numbers which i'd say are ballpark. Potential lift per area is almost the opposite at about 1:5 as lift increases with the square of windspeed, so maybe they are a lot closer in terms of efficiency that I thought. However, there are so many assumptions in that 'calculation' that it's laughable so i'll just go with the boat speed/wind speed ratio

Or 'MI goes almost the same speed in a shirtload less wind'
Both incredible achievements either way, just to me MI is more impressive, personally. No factual justification behind it.