I was thinking more just the hulls/beams, and not neccessarily the rig. Although a flexible set of blades/rudders doesn't sound too appealling either.
Keep always one hull out of the water, than you don't need to care about stiff beams.
As an aerodynamic specialist, I can tell you that you can neglect the stiffness of the boards and rudders. Catsailor are so stupid and mount tons of lead on tip of a daggerboard.
So it boils down to hull stiffness, which is not a problem for beach cats with sandwich skins. I remember my Dart 18 and early Ts were a bit flexible, but nevertheless fast boats.
Cheers,
Klaus
