aluminum pipe SHOULD be used, but many people use regular old exhaust pipe. Why? Because its cheap, easy, and can be bent by any local muffler shop. It can also be easily welded to for things like BOV's or extra injectors.
And for peak power larger turbo's definitely help, but for off idle power that little t3/t4 will pull like a diesel. Something to consider on the street.
ON A STREET CAR smaller turbos definitely have some advantages.