Another view point from Europe!
The composite boat being talked about is the Gui-Gui Prod in France, anyone who was at the Europeans this year would understand the push.
The feature was a small and relatively flushy feature, some did well, plenty did not. What was clear was those in the Gui-Gui appeared to have a significant advantage, almost to the point that those on the bank were discussing will a different 'formula' emerge.
The hard thing is, all these guys were international standard paddlers, so they'd rock in a bath tub - however they were going larger and flushing massively less than most others. my feeling is that this is not so much to do with the weight, but actually the stiffness. I know when I twist I can slightly distort the plastic on most boats, so I'm guessing water subtly does that too when you plug.
Also the outfitting was very good, simple, light, but very, very tight.
So combine the two - a boat with little/no flex and you completely at one with the boat the difference will be considerable.
I paddle a Jackson SuperStar and love it, I'm as tight as I can get - if I used the outfitting these guys were (lots of foam blocks wedging) I'd improve slightly, but I'd still have the plastic flex.
Also, I think because of the 'strength' of plastic, we do abuse the boats - look at the rails of any playboat, I know mine doesn't look like EJ envisaged them! That has to make a difference 5%? That the carbon won't - because they'll be looked after differently.
That said, don't forget these are differences that the top, top end guys will notice the benefit of - and I saw these boats paddled by international standard guys, won't make any difference for me!