Piros said..
we think all Sups need to be light to work. Maybe on the really small boards but boards 9-0 and over it's not an issue.
Even on very small boards, weight adds more drive tough the turns. Extremely light boards will stop dead in their tracks in case of rider mistakes... or chop.
Cannot find the reference, but I read once that some ultra light shortboards were tried (Salomon? Aviso?) and found unriddable.
Whatever the board length, light is great for aerials and other acrobatics, and very tight turns (whipping back the board at the end of tight cutbacks), even for longboard shapes if you envision a "performance longboard" style, but for less extreme use, some weight makes for a more pleasant ride.
In my favorite 6'8" / 6'10" range, my boards are 7.5kg with pad and fins and I find them OK. 6.5kg is nicer for tight turns, and I had a 9.5kg once that liked a lot: was great for open face carving turns but harder to control in the pocket.