How long would it stay stiff?
depends on how many blue pills you take
but seriously, the general laminate on our boats is so over specced (we use foam that is almost twice as thick as the big builders) that the boats are quite unstressed. We can do this because each boat is built carefully by highly skilled people and there is no mess left inside the hulls and the joins etc are all carefully prepared and finished. In the 166kg build there is really not much difference in the overall laminate. We just put more in there because we had to get the weight up!
I'm not saying that I want the weight to change for the class (i'm in enough trouble with the class already!!) but if we want to reduce cost and improve the longevity of the boats there are some solutions out there.