I have always seen boat stiffness as a field of diminishing returns. Each progressive step (increase in stiffness) results in less gains and at some point the additional increase is simply not worth the effort (cost).

Therefore, it is my opinion that the best approach is to find a good optimum here. One where the stiffness is sufficient to provide for a nice sailing feel and good performance without going overboard with respect to say weight or costs.

Just to give an example ; a 180 kg F18 can be made very stiff and perform well with that but I doubt that any such stiffness increases are enough to results in more performance then say a slightly less stiff but 30 kg lighter platform of the same dimensions.

Boat design is always a game of balancing one benefit (or drawback) against another. The designer who does this the best will produce the boat that performs the best over a wide range of conditions and thus be most competitive in the long run.

Wouter