It depends on how power do you want and of course what turbo you have. For the stock turbo 70mm exhaust pipe is fine. Even better without cats and a free silencer. If you are planning for high power with a big turbo 76mm is ok without cats and free silencer. In this setup I made I have 76mm pipes with the stock cats and a silencer at the end for a quite trip. When the throttle is open the gases are going to the easiest and more "free" way without cats even a silencer. Just an 76mm pipe