RE: tuning by sandpaper
There are a lot of factors involved here, but in simple summary, no, there are no surface treatments or processes in hi speed (planing) windsurfing that will "dramatically" reduce friction by anything anywhere near 50%. No magic bullet.
In planing windsurfing mode, the use of small directional surface scratches (400 grit, directional) theoretically allows a very fine layer of water molecules to "bond" to the bottom of the board surface, thereby producing a "water flowing over water" situation, which in theory, in this mode, delivers the lowest effective coefficent of friction.
It's also worth adding to my above comments that the bottom surface finish on current / recent production Starboards, and the surface finish of the fins is optimised to an efficent finish right out of the factory (especially the hi performance boards and fins, where a "perfect cosmetic" (visual) finish or a shiny polished look is compromised by a directional sanded finish - often referred to as "pro" finish or "whitewashed" ).
Yes, like most things there is a small potential to spend a lot of extra time and detail and marginally improve on the original, however (at a practical level) the importance of these gains also relates to the conditions of use: at 45kts and going for 50, every little detail counts - especially considering the exponential nature of drag. Most planing windsurfing occurs at speeds of 15-20 kts, where any gains achieved are much less significant. (and especially when balanced against the very real risk of potentially damaging the product or reducing performance if the tuner is not so expert).
In non planing activities ( where hull speeds are much lower and operate in displacement mode, Serenity or America's Cup .), complex drag reducing treatments, coatings, or skins have been shown to have some performance edge, but irrespective of any claims made, the actual on water improvement is also marginal, rather than anything near "dramatic".
(However, in America's Cup, even the smallest margin is "dramatic". A bit like their budgets.
Which is why they moved to ban such coatings in 12 racing. But you can be assured every aspect of those hulls are faired in very, very nicely indeed.)
Cheers ~ Ian