I added an 05 Coupe mid-pipe to my 05 Sedan.I paid a total of $120 to get one from CA.
It increased the sound slightly, which I really liked, plus it gave me at least 1 MPG more.
Fairly ez to install. I bought a new front gasket from my local Nissan dealer for $9 and reused to rear doughnut gasket torquing it down 5 pounds more than spec to ensure a good seal.
On my 05, I removed the cross bar You can see it in this pic. The G35C mid pipe curves up and slightly touches it. Others have bent it. I just removed it...saves weight too!
Sedan vs Coupe
Here's what it sounded like on my Sedanhttp://
www.youtube.com/watch?v=NIF-GD-WaH8