I'm assuming you don't mean a close reach, but well off the wind? I don't think you'll get a definitive answer, but 20 might be achievable.
The reason that top-rated windsurfers can "only do 30 knots" where they live (and not the ~50 knots of the world record) is wave action.
Sailing's world speed records are set on a stretch of water in France with a long sandy "spoils" beach that keeps wave action down in strong winds. (With a shifting sandy submerged leg you absolutely have to miss or get a bad crash). A similar spot is in Australia, where the latest multihull records are sought (also ~50).
I don't think you'll see the 30 knots of windsurfers, but have a cameraman handy when you try!
I own a Navman M300 for 2 months now, which is a GPS receiver that gives me speed, average, distance, lap times and so on. My personal best is 20 km/hour (yes i'm Dutch!) which is 11 knots. I think i can top that, because i was not able to sail with *real* windy conditions yet. I will keep you informed!
It's fun to speculate about this issue, but to make a somewhat scientific statement one needs to define the distance over which speed is being measured. For instance, Blackburn on his historic Laser crossing from Tasmania to Australia measured a momentary speed max of 19 knots, GOING DOWN A WAVE. But under 'more normal' conditions, Lasers max out at 15 knots at best, while reaching in heavy (25 mph+) winds. And Lasers are a bit faster than Sunfish (if you can keep the stick up); therefore, I think that the above statement (from moredownhaul) is roughly correct.
Another issue to consider:
Even though GPS based devices are supposed to be reasonably accurate, trials have shown that different brands give different results. For more info, consult the foiling Moth website of Rohan Veale.
Heavy winds don't always mean more speed. Fish like many sailboats after a certain wind speed had to depower the rig to keep it upright. I will say that on a plane on a beam reach a Sunfish will outdo many displacement boats.
Where I used to sail we had a variety class where all boats started at the same time and were scored on handicaps. Give me 10 knots of wind and I was up with the best Flying Scot (rated 91 versus Sunfish's 99) in the fleet. But let the speed increase to near 20 and I couldn't keep the boat down without depowering and the heavier boats didn't need to depower as much and were definitely faster.
The running gag for the fleet was if the Hobie Cats weren't home and derigged before a fish crossed the line they knew they had lost...LOL
I once came up with 14 mph. When my bow crossed a crab pot marker I started a timer on my watch. When the stern went by I stopped the timer, and then I just worked out the speed from velocity = distance / time. Obviously this method isn't as accurate as using GPS, but it still gave me a rough idea of how fast I was going.