The theory with exhaust is unless you are making 700+ the best setup is 3" expansion pipes with 3" to 2.5 test pipes and a 2.5" catback. The flow of the exhaust gasses cool and contract so to maintain the velocity you decrease the cross-section of the pipe. That's the theory, I've never modeled it nor do I have any dyno info to back it up, I'm sure Z1 could provide you with such data. Think of the system this way: at stock power levels the flow requirements are optimized by the factory, when increasing the flow requirements (how much the turbo pushes out) your choke point becomes those portions of the system which are not ideally sized for the requisite conditions, thus by increasing the size of the exhaust and removing restrictions (like the cats) you allow the exhaust to breath OUT better. As for the intake, you will see the biggest gains from doing the intercooler and piping upgrade
https://www.z1motorsports.com/z1-produc ... -6555.html . Again, the discussion is all theoretical based on thermal dynamics principals, what it all behaves like in the real world is a question you'd have to ask the vendors who have done the research.