Interesting perspective....so what it really breaks down to is what the control system is capable of. We probably won't see a 60' windsurfer because they're already terribly unstable - especially at those speeds! There is not a mechanical system that can respond quickly enough to keep the craft on it's toes...there's barely a human that can directly control a 10' version (or whatever) in the wind required to get it up to those speeds. The windsurfer is very refined to match the size and power to what a human can control but it's far too twitchy for a mechanical system to handle so it's size and efficiency is pretty limited.

The next problem is finding lower water resistance (smooth water and foils) and adequate wind power. If you can be more wind efficient and go faster with less wind, your chances of finding smooth water are much better - not to mention finding weather windows of opportunity is much easier as well. The foil thing is becoming a pretty solid limitation in that most of the existing foils are having a hard time maintaining proper flow and lift through the water at 50+ knots.

So the next step to go faster than windsurfer is to build a more efficient craft - but it has to be larger - which means it's outside of human strength realm leading to mechanical control - which means it has to be more stable for the mechanics to handle it. This has lead us to the designs we see with MI and Sailrocket and they are still struggling with the foils. Maybe a planing version of these guys with a vertical fin (in the spirit of the windsurfer) might be the ticket? They'll loose some efficiency and have to sail in stronger breeze - but maybe they can maintain control.