ya kwongs civic post confuzizzled the hell outta me unil his other post, then it made sense. and i dont wanna get involved here in this but let me clarify some "exhuast" knowledge for ya:
on a N/A car, your best bet is to stay as close to stock size piping as possible, usually 2.5" or less. dont bother getting high flow cats unless itsl ike a car from the 90's at most they take 1-2 HP.
a turbo car on the other hand, will benefit the most from a 3" straight back pipe with no cats and bends. he difference between a 3" and a 4" pipe is sooooo minimal in resistance t airflow that 4" isnt trully useful until around the 600hp mark. think of it like this : air resistance to 2.5" pipe = 3, 3" pipe = .650 , 4" pipe = .200, you see thats why it doesntm ake much difference.
but in the end with a turbo car, just run an open downpipe, faster spoolup, sounds way better. or do what some people do, they make a y pipe and weld it to the end of the downpipe, and have a straight through 3" pipe with a muffler, and they have an electric switch open a cutout in the other side of that y pipe so when used air goes straight ou the downpipe again. quite and smooth, vs. loud and powerful.