I believe with the 1st-gen twins, replacing the plastic with aluminum intake manifold has allowed the engine to produce 5 more horsepower. That’s the only advantage I see. Aside from that, it just looks more durable.Why though? besides looks. It's heavier, heat soaks easier, etc. It's not like aluminum somehow flows air better. And being NA, holding pressure isn't a concern, though as far as I know the WRX plastic manifolds never had trouble. Infact the 08-14 plastic manifolds flow better than the STI aluminum ones.