I agree, many of the speeds over 37 knots were set outsite the ditch.

The 38.86 Pascal Maka record was set at Canaries Island and many speeds over 40 knots were set in the ovean with very strong offshore winds (Bjorn Dunkerberk., Olivier Auger)

Also, I remember in 1990 that there was a race between a windsurfer Pascal Maka recordholter at that time and the 60' racing trimaran "Haute Normandie" over a nautical mille course (approx. 1850 meter). The windsurfer had the advantage on a downwind reach, the Trimaran had the advantage on an upwind beat and speed was equal on a beam reach. The speeds recorded were at 28knots but the race was setup in open waters but in fairly strong wind (30knots+)

As far as the speed of a beach catamaran, it would be interesting to see a bigger cat with a smaller rig in strong winds.

I am certain that a big cat like a 20fotter with a small rig could go very fast in protected waters. I have a video of a Tornado event where they setup a course and the fastest time recorded was 27mph. I am certain that they would have gone faster with a smaller rig because they were overpowered.