I reckon HALF the record speed on a standard beach cat over 500m would be optimistic. Boards are designed to be "almost perfect" speed machines. The stronger the wind blows, the more the sailor pulls the rig down onto himself, creating lift on the board. Biggest problem these guys have is trying to keep the whole thing on the water at these speeds. Cats have the opposite problem : the stronger it blows, the more the leeward hull is depressed into the water, creating more drag. 3 big fat guys on the wire will only increase the downward force on the leeward hull.
Anyone who says he averaged "30 knots" on a cat over a 500m course was either on Playstation, on Hydrofoils, or on drugs.
Me, I heard the news, and rushed out to buy some windsurfing kit. After a 13 year lay-off from boardsailing, it was a blast. You CAN have the best of both worlds !
By the way, check out Dave White (son of Reg, Tornado Olympic sailor) - Fastest production board sailor on a F2 Sputnik. Go buy one, and go fast !

Steve