If by metal you mean aluminum then those are one of the worst kinds of intakes because after driving awhile the aluminum starts sinking in all the heat from the engine bay and you start getting warmer air into your intake and that defeats the whole cai purpose
Think of it like a push pull kinda situation, the power wire is pulling all the 4 guage current and is trying to dish it out to the ground but if its smaller theres alotta resistance there.
I sometimes even make the ground larger than the power.