With dolphin striker or not? Makes a huge difference. If you have a dominant load direction you would go for box section. Since you want not get injured or cut your trampoline, take something with round edges. CFRP should not be build in to tight curves anyway. Common problem, most people think it is material only, but each material needs it specific design.

Apart from a sagging jib, I am still not convinced why I should look so much for stiffness. I agree about the better feel (see my very first post). But as an engineer, I don't have a problem if one hull drops 40 mm or so, as long as I keep one hull out of the water.
For the jib, I would look for stiffer rigging (i.e. rod or dyform) and stiffer front hulls, before I look for the beams.

For the lost energy theory: the energy loss due to flexing structures is lower than 1% (if you bend something you get 99% or more back). A spring in a watch is a nearly perfect energy storage, ask yourself why. So from the little bit energy which is stored shortly in the structure due to bending, you loos only a liitle bit. A little from a little bit is how much?
The best way to improve speed is to concentarte on the import thinks: Skill, sail shape, foil shape, hull shape, deck layout.

and btw, I like the flexing wings of the B777. It saves tons of structureal weight.

Cheers,

Klaus