OP I get excellent torque, good flow with HM / Perry / HSV / Walkinshaw 1-3/4 Tri-Ys, 3" modded cats and a mild steel 2.5" X-Force.
Performance gains are solid, sound is tough down low, lets the killerwasps loose up top and is silent on AFM (no drone).
[video=youtube_share;jE2Wg1B9hDg]http://youtu.be/jE2Wg1B9hDg[/video]
NB: All up it will cost you $1200 for full install including work to the std cats (3" inlet and outlet), and fitting J-pipes at rear.
Pisser is that the stealership put mild steel pipes on e85 car, you should add $500 for stainless.