The video of the tornado shows very gusty winds. tough to sustain a steady speed. My contention is that beach cats without hydofoils will never go much faster than the record set by Hobie Alter on a 16 in huricane gulch -circa 1971 or so. Maybe in a speed ditch you could get a cat to add more than a few knots of speed, but not in open water for 500 meters. For efficiency the best wind is barely single trapeze weather. Drag is at a minimum and apparent wind is kicking in.
On another note i think Finnian set the record because he finally got the perfect board shaped by Richard Greene who knows what to shape and makes it come out perfect. also it was great conditions.
The other thing is a radar gun or equivilent is a good way to measure speeds.