Looks like the one from the GT dealership is real judging by the link to Opera Performance's website. Shame it's in Japanese, though. It does specify in the info section of GT6 that the tuning menu was performed by OP and the suspension (I think) by Amuse with the help of GT (hence why it's neither in the Amuse or OP dealerships)
The '04 Opera Performance in the game has different alloys to the one shown in the link above. That one looks like the alloys of the 350Z RS in the game. Also, I don't think OP's '04 model was actually called the 350Z RS, it was just 350Z.