Hi
I can not answer that one...
As IMO are the max sized ones not the ones you can ride in the lowest wind - but they are a lot faster up and downwind in lower wind.
Others would say they ARE the best
The Sonic2 18 m2 is not sufficiently light for ultra marginal wind IMO.
But some race kites has max peak power, which is essential for ultra marginal hydrofoiling.
If too low aspect, too fat, they can hang if they are ultralight material (max 27 g/m2), and they are a lot easier to handle, dont bowtie in odd situations, and turns faster.
BUT, you lose the peak power needed for getting up and going.
Race kites can have difficulties hanging in this low wind, so tricky, but IF you get up and going, they can be the holy grail.
Although, some race kites has a profile so they fly to the edge and lose the power, if you dont have speed or wind - and both are present for racing.
Not so for ultra marginal wind foiling.
Also, a 13 or 15 m2 turns okay for the average weight, not fast, but okay, whereas the 18 and 21 m2's does not.
Long lines is a must, 30 meters or something, and a big wing quite high aspect - otherwise it does not work at all.
Again, opposite to racing, where even bigger kites on shorter 14 m lines are typical.
For the "Fun" factor, 12-13 m2 for the average weights, are IMO and experience by far the most fun, and can go almost, or just as low as a 15 or 18 m2, but WAY more fun.
If you are over average weight (over 75-80 kg) you need a bigger size of course.
Peter