just a fwiw I didn't really have the $$ to spend on a Corsa system. So I looked around and found these guys:
http://www.classicchambered.com/classic/products.html
I was going to buy two of these and cut out the OE 90 resonator and splice two of these in it's place and keep the rest of the OE exhaust on my 90. I don't know how loud these are, but I was thinking these parts reminded me of my factory side pipe mufflers on my 72 LT-1. No restriction, or not much going on with these, and I figured going thru the stock mufflers the noise should be kept to a tolerable level. Don't know if this helps you out or hinders the resolution of your search for flow and no noise.
Tom