Firstly there is no such thing as over stiffness in a catamaran platform. Any one that sails hobie 16s at the top level will tell you that. The h16 platform is bendy no matter what you do to it but it has been proven to death that a stiff 16 is a fast one. Don't bother rocking up to an Australian h16 nats if your boat is not glued together and your tramp is not as tight as you can pull it. You'll be lacking boatspeed if you do. Basic lessons about catamaran sailing need to be learned by a fair few of you guys.
First of all, are we taking advice on boat design from Hobie 16 Single Manufactorer One-Design sailors ?
Many designers still scratch their heads over the "unique" design choices made with that boat.
Other then that your example only explains that a badly designed platform with unbelievable amounts of flexibility is not what we are looking for. Not that "overstiffness" is impossible.
In fact ORMA tri designers and those of larger offshore multihulls tend to disagree with you and your H16 sailors. Now, please don't make the error of putting up the strawman argument that these designers are saying to lots of flexing is good, they don't. They are mere saying that overdoing it on the other side of the spectrum is not desireable either. As so many times in life the optimal point is somewhere in the middle of both extremes.
Other then that your argument resembles the situation where a man who is exhausted from thirst is best helped by thowing him into a large lake. I dare wager he will die either way. If one thing is "not good" then chance are that 100% of the opposite is not good either.
Wouter
