So if you build this ultra stiff all carbon uberboat, how much faster will it be? It would still have a similar water line and weigh as much as the old T 4.9.

There are ways to deal with this issue in a positive fashion, rather than letting it hang over the class' head like some storm cloud. Personally, I like the idea of a "prototype" rating: one off boats that are eligible for participation but not necessarily scored in official standings. You could implement that along with a GTO rule to ensure that the entire fleet does not become obsolete overnight.