IMO and experiences, the short version:
The light rider has benefits when only limited power accessible, meaning in light wind.
When windy, the heavier rider is faster, as he/she can transfer more wind energy to speed, and has better control at high speeds.
Regarding "carbon is faster", yes, but not because it is lighter as such, but because the swing weight when oscillating is reduced, thus less power loss.
Besides the fact, a stiffer board means you can transfer more energy to forward power (=speed).
Having a higher weight centered mass is key, for speed.
When turning or accelerating (same thing almost), that being a formula one car or an aerobatic plane or a surf board, low weight is much better.
But for max speed, not so.
The heavier rider needs more lift from the foil wing yes, meaning more drag, but has also more lift from the kite, which somewhat evens out, besides the fact he/she has more forward thrust from the kite.
And heavier riders can control higher speeds, just a fact, and why the speed records in high winds are always heavy riders.
This also goes for TT boards and windsurf boards, same thing.
Regarding gliders, it can not be compared whatsoever to the wind driven sports.
For full size gliders we use ballast as it will increase flying speed, and competitions are almost only about covering a huge distance in the shortest time.
Glide angle is the same for light and heavy gliders.
BUT, even if it was only about coveríng distance, there is always some wind present, and suddenly a light glider is useless, if the glide speed equals the wind speed, you are going nowhere againt the wind, you are just stuck in the very same place
For RC hang gliders we use weight to go faster, A LOT faster, we more than double the weight of the gliders, if windy.
This is where it is not equal to kitesurf, as in kitesurf you have trust from the kite, meaning bigger kite equals more thrust (and also more drag yes)
With gliders the only thrust we have, is the gravity component.
Meaning, if you quadruple the weight, you go twice as fast, when no wind, a simple fact.
Even if you dive straight down towards the ground, because drag is squared with speed.
But when wind, you can actually fly backwards relative to the ground, if too light, where you will fly forward if heavy.
When turning, equals acceleration, weight is loss, but as the speed is so much higher when heavy, you still outperform the lighter glider hugely.
A bit off topic maybe, or maybe not, but you can not compare to gliders, nor can you compare to the advantage of lighter and stiffer "vessels", as centered mass is a very different thing, and has many benefits, depending on the conditions and task