I just recently put a full dual exhaust system on my car. Its the full race spec Kakimoto racing Full Mega N1. 75mm each pipe from each turbo all the way back. No cat factor.
I can certainly say there's no drone anymore just pure "race crackle" if that makes sense with each gear change esp after 3000 rpm. I havent put it on a dyno, but what I have noticed is the turbo response seems to be much more punchier and revs much faster. Also love the Kakimoto quality stainless steel all the way through, that much nickle content a magnet can't even stick to the pipes.
No trouble from fun cops as of yet, though I drive pretty sedate unless a wrxy boy or a commonwhore comes along.
These are Mine, which are now discontinued http://www.nengun.com/kakimoto-racing/full-mega-n1-full-dual