@mh.#30572 sorry, yes missed this one - indeed the PWA riders are usually totally tuned up on the 97 size, which they need for staying planing and to be able to use 7.0 down to 5.0 in case of gusty winds - so they may have a 80 0r 90 for GPS, but rarely use it - also at Defi I know from own experience, a too narrow board with the figure 8 course is super hard if the wind switches slightly/you are in bad winds of someone else, then you are always pinching upwind and a bigger board helps. Like that you can also use a slightly smaller sail (in theory!).
For pure GPS, for sure the smallest possible to get the best top speed, to a certain limit - so 90 or even 80 should be the right answer there, pure downwind speed is very different to Defi long distance with sometimes beating upwind, or slalom having to keep planing at every gybe/guys are all 90kg+.