I recorded 15.8 on a stock Wave this past summer and I was backing off a little early. I was blowing the sheet when there was about 1/2 inch of bow above the water. Wind was quite variable as it is a lake in VT. However at 1800 feet above sea level we do get good breeze a lot. Top winds were probably 22-25 in very quick gusts. On the same lake I have done over 25 on my sailboard. Great part is there is a 5 mph speed limit for power boaters! I have a Garmin GPS that is meant for a car but I put it in a water tight bag. There is a function for saving the highest speed so I don't have to actually see it while sailing. I usually look at it when I get to the far side of the lake.
I recorded 16.4 mph on my stock Wave.