Obviously not being an expert, I can only give you a short answer. This is also why I started the thread in the first place.
I have seen a few pictures of flying A-cats, where it really did seem as a problem that the platform was too light. However, a lighter platform will require less force to drive it through the water, decreasing the all up loads on the platform and rig and therefore making it easier to control even in a big breeze. Still, when you go windsurfing and the wind is picking up you go for the smaller boards. It would be natural to conclude in the same for a beach cat. Hoping for some good answers on this one.