WIND... how much can we sail in?? 40 knots of wind is a lot, when sailing any craft driven by wind. Most yachts will be heavily reefed already. Worst thing is the sea state though. 60 knots steady is close to a hurricane. I've tried steady +50 knots of wind gusting 70 to 80 knots (officially measured) in a 28 foot sail yacht on open sea - ITīS HELL!!! The boat was knocked down, cockpit filled and simple jobs like opening a hatch became a fight for two men.
Handling a kite or windsurfer in 30 knots of wind is only a job for the very best (but surprisingly possible), and the wind pressure on anything - the board, your body etc. makes it a struggle.
On flat water (quite theory) I guess I would be able to windsurf in 30 knots steady with matching gear, but the fun factor is probably limited ;-)