I have a '12, Berk High Flow Cats, I did have a non-resonated x-pipe, but traded with an amazing team member for his 12" resonated FI x-pipe, it changed the car for me. I dreaded driving it before, it was just way too loud, I'm in love with the car now, it sounds like a Lamborghini. The 12" resonator removed a lot of the drone in the inside of the car, and left most of the sound outside the car (but you still hear it believe me).
I've read through a few reviews on the increases in HP and Torque, I know the system I have makes an appreciable difference, I believe it's about a 15 HP difference.
370Z "TDX" Cat Back Exhaust System, Fast Intentions
They ran some Dyno runs on the exhaust towards the bottom. Regardless it makes my car sound amazing.