intake to breath in air with minimal restrictions (your 2 1/2" intake piping and less restrictive air cleaners), then you want the engine to exhale that air/fuel mix as least restrictively as possible (an exhaust consistent with the intake, ie. MSP exhaust manifolds and an exhaust system consistent with that). A 3" downpipe, 3" to 2 1/2" midpipe to 2 1/2" exhaust is consistent with the "breathing in and breathing out" of the intake and exhaust events. With stock turbos, it is not necessary to do a full 3" exhaust. But to optimize your system, a 3" divorced wastegate downpipe tapering to a 2 1/2" rear system (ie. Catback) will maximize the performance of your system. The intake, engine dynamics, and exhaust are a system that create overall performance. Putting a stock exhaust on the previous components is like using a C-pap with a restrictor in your nose.