http://www.vwwatercooled.org.au/f28/diesel-exhaust-flow-calculations-42772.html
This link and the link within it are worth checking out. They relate to diesel rather than petrol.
Chip IT quote 151hp and 573Nm for the 3.2, without a boost increase, just the chip on setting 4/5. Run the boost increase as well and that rises to ? (has to be 170-180 or so?), which is getting close to the 200hp = 3inch exhaust theory.
I'm guessing the Chip IT guys supply a 3" exhaust package because it works well, maybe for the above reason.
I'm ordering a 3" Scotts Rods system today and I blame Daryn for the decision.