Wouter,
You are quite right that windsurfers will not perform to their maximum in choppy water, I windsurf at the same lake that our speedsailing league is held, and the top guys at speed are unbelievable, when I am backing off and getting airborne, they are locked in and flying. But they are on the right equipment (F2 Missile, Naish Stealth sails), and one of them holds the African record, the other holds the kite world speed record. I see from the SA speed league site http://www.windsurfingafrica.org/GPSSpeedLeague/tabid/834/Default.aspx
that they have 10sec, 100m, 250m records, so not only measured over 2sec, my mistake. At these speeds you will sail 100m in 5sec, I just don`t think you will see a beachcat do that unless it`s on it`s trailer.
What the windsurfer guys have to worry about is those pesky kitesurfers getting faster, and faster ...
With regard to the tri vs windsurfer video, I`ll agree, no doubt the tri was not making the effort, looking at the water and windsurfer rig size and board choice, it would have been around 18-20knots windspeed, no white horses, very little chop. My guess would be their average speed was around 22-25knots in those conditions, which is consistent to how fast they appear to be going - 40knots looks a whole lot different.
And yes, knees make remarkable shock-absorbers, our creator was the first engineer, none have come close to replicating anything that came off his drawing board.