Select to expand quoteBellerophon said..
extract of www.windmag.com/actu-nicolas-goyard-titre-mondial-iqfoil-belle-recompense-travail-effectue-interview
"On the physical part, after Israel, I was at 90-91 kilos and I knew it would be a lot for iqfoil so I gradually lost weight to reach 87-88 at the start of the competition and finally 86 on the last day! "
More about the weight gaining : www.letelegramme.fr/voile/pierre-le-coq-en-iqfoil-il-faut-prendre-15-kg-16-08-2021-12808052.php
And here is his brother Thomas , former RSX sailor about his weight gaining from 73 to 92 kg to be competitive in IQ foil www.sudouest.fr/sport/voile/iq-foil-le-secret-de-la-planche-a-voile-volante-par-thomas-goyard-10732325.php
*The weight. Yes, you have to grow, I went from 73 to 88 kilos since Tokyo and I live 92. It's not easy, even if I starved myself to stay at 73, it was not my natural weight. So I did bulking, weight gain. To gain so quickly, it's not just muscle, there's fat too, and it's hard, I've lost my breath, I don't climb stairs anymore"
The same article explains how former olympic champion Charline Picon in RSX class (gold in Rio / silver in Tokyo) did not switch over to the IQ foil since "she would have to put on too much weight to be competitive."
An 'athlete' deliberately getting so fat and unfit they can't climb stairs so they can be competitive? 
I have no doubt that if outright speed is your goal a weight vest will help you go faster.
However, as others have said, it wrecks your back over the long term. But it is also only bearable to wear in "speed-like" circumstances, i.e. for straight-line sailing over 500m (or whatever the distance/speed is that you're trying to beat), so *not* around a course that requires jibes/tacks, starts, etc. (presuming class rules even allow you to wear one).
That said, it would be an interesting test; I reckon it will bump your straight-line speed instantly by a knot or so, maybe more once you get "comfortable" using the extra weight. The control gain over your kit at the margin is quite remarkable.
Select to expand quotesboardcrazy said..
An 'athlete' deliberately getting so fat and unfit they can't climb stairs so they can be competitive?
You should see American football players. They put on a lot more than that. Even the fittest and leanest are 85-90 kg and it goes up from there. I long campaigned for weight classes in American football so that everyone could participate at the pro level but to no avail (j/k).
btw, I think that it's a bit a lot of hyperbole that TG gaining 10-15 kg leaves a very fit athlete unable to climb steps. If that were so, literally nobody would be hiking and camping in the mountains. I carried at least 15kg at altitude on a hike/climb (3000m) - reasonably fit but nowhere near Olympic level - older and smaller guy. I was about 73 kg at the time.
I lost 10 kg about a year ago. Totally wrecked my quiver and made it hard to carry my largest sail at times. Basically, I had to scale down a size. I guess that ends my Olympic dreams... 
(That plus me being 35-40 yrs too old). Having done essentially the reverse of using a weight vest and being slow before the wait loss, I'm now just concentrating on staying faster than paddleboarders. 
