Hi, sorry not to reply sooner.
The difference between ram air kite (I mean big ones) in light wind is at different points:
Ram air kite, in very light wind can have better upwind ability. This is what we see in the french racing contest, but when you go down wind the kites are very difficult to create apparent wind (as the acceleration of such kite is lower than inflatable ones at the same size). And the turning ability is totally different.
The Notus 21m is super stable with 5knots of wind. I mean you keep the kite at the zenith and without any movement it stays in the air. This is the only 21m inflatable kite I know that do this job.
With wind lower (3-4knots), you need to moove the kite to keep it in the air.
With a foil board, I can start at 4.5-5knots of lulls. With lower winds, I can't launch my foilboard.
Additionnaly,I never used with a SUP in low wind (4knots) I'll try to make the tests, but these days wind is 35knots here
As I detailed, power is square of the wind speed and increase with the kite size for a steady wind. Then you can ride with a smaller kite (specially with a big board as a SUP). But the power management will be different.
What I say is for example, with 5knots of wind and my foil I can ride with both 17m and 21m.
But with the 21m I feel full powered, and I can pass lull, with a 17m this is more tricky.
You always have to remember that when the wind changes from 5 to 4knots, this is 20% less wind (this make almost 1/3 loose of power for the same kite size, kite size difference between 17m and 21m is 23%)
You can ride with different kite sizes but if wind low down, with a small kite, you fail.
Additionnaly, when you want to go downwind, a bigger kite is definitely faster.
Why I use a foilboard instead of a SUP or a raceboard:
The reason is simple I kite now for about 15years, I windsurfed a lot. I never love lightwind.
But with a foil board, the feeling is totally awesome in light wind for different reasons:
- feeling of ride: no noises, no chops
- unbeatable upwind ability
- tricks to do strapless