Good few years ago now Nick Green (a mate of Dave Whites) won a round (perhaps overall ?) of UK speed championship on an F2 Axis. He managed 41 knots on it and was only using twin cam sails !!!
Perhaps its more to do with angle off wind and flat water rather than Slalom or freeride but reckon Slalom would walk it back up course.(ie at angles less than 90 degrees)
Again ,a few years ago a bloke won monthly speed challenge at Sotavento hire centre on an F2 powerglide; his speed was 41 mph. !! (37knots)