I have a true dual setup with 2.75 inch piping all the way back with test pipes and magnaflow resonators. If you don't want it loud DO NOT do this setup or anything like it. not sure on performance gains but i've messed around against a stock Z and pulled him by a half a car when i just had the exhaust. the sound compared to stock is not even close. I think it sounds good but loud but i'm sure others would dissagree.

 

Anything above 2.5'' would be extremely loud and stupid unless your FI. I have the 2.36'' or 60mm HKS and it seems to be just right.

 

today I've decided i'm gonna stick to my plan and go with the custom 2.5" dual exhaust. If it's too loud I'll just put two 22" resonators in.