Although weight is one factor that determines performance, more important factors are profile (ie tear drop vs air foil vs round) and bend characteristics. With that considered, a given alum mast may actually have superior performance to a given carbon mast. If profile and bend characteristics are equal, a lighter carbon mast would certainly be desirable. Carbon allows more flexibility when engineer bend characteristics since the cloth and thickness can be varied along the spar. But then the factors of cost must come in. Also, aluminum masts are generally lower maintenance and more durable. So the alum v carbon is not necessarily clear cut. I don't know specifically why nacra is switching, but I bet cost is a big factor.