I would just like to state that hydrodynamics is different than aerodynamics. There are so many other factors in hydrodynamics that to compare the two is just silly. Unless you are building your own hulls, don't even consern yourselves with it.

Are speed estimates exaggerated? As long as humans have egos and pride. At our beach we have where we offer rides for the public on Hobie Getaways and free speed measurements via GPS. Here are my personal findings of top recorded speeds on a few different boats on close reaches @ downwind without spin.

Hobie Getaway: 18.5mph
Hobie 16: 22mph
Hobie 18mag: 24.6mph
Hobie 20: 26.8mph
Nacra 570: 24.5mph
Nacra 6.0: 27.9mph

I am sure there are production boats w/spin. that could break 30mph downwind. I personally think the speed of the boat has more to do with who's sailing it and less to do with hull design in everyday use.