A 3" exhaust on a turbocharged engine will be good for at least 750BHP, so its not likely to limit an N/A engine,
An N/A engine needs an exhaust system that designed around the engines pulse characteristic's, whereas a turbocharged
engines biggest restriction is the turbo it self, so anything you can do to make the exhaust after the turbo flow more freely will help power,
so doing the same to a N/A engine will just result in disappointment.
I personally wouldnt agree with that, i ran a 3" system on my Rx7, it was running 450bhp on low boost, over 500bhp on high boost, still only 1.1-1.2 bar, i swapped to a 3.5" system, the turbo spooled quicker and result in a few more bhp, had i gone to 4" i would have noticed similar improvement again
http://www.exhaustvideos.com/faq/how-to-calculate-muffler-size-pipe-diameter/
+1
I ran a 4" system from turbo back on my rx7 helps the turbo spool faster.
There are plenty of people that run 5" systems on turbo cars