This has been done to death before, but "Hull Speed" is a bit of a red herring when it comes to cats. The idea of "Hull Speed" is a rule of thumb that applies to hulls that are primarily limited by wave drag. Beach cats are primarily limited by skin drag. So, whilst you can adjust the constant in the "hull speed" formula to something appropriate for the shape of a cat hull, and that will tell you something about the wave drag of the hull, it still doesn't give you a limit on the hull speed because wave drag is not the dominant factor.

To answer the original question, I think it depends on the conditions as well as the boat. I haven't sailed many different types of boat, but my experience on F18 and F16 is that in flat water with a steady wind the limit will be righting moment i.e. you won't be able to hold it down. With waves or gusty weather, it'll be bouyancy in the bows that's the limiting factor. Then again, flat waters with very strong, steady winds are hard to find, and normally we're chasing VMG not straight line speed, so this may not be correct.

Paul