Unlike the 1st gen G, which we all had down to a science, there is so much speculation in this area for the G37 (Coupe and Sedan) that its hard to make heads or tails out of anything related to the exhaust.

Couple of threads which bring this point home:
-https://www.myg37.com/forums/intake-and-exhaust/246788-oem-coupe-midpipe-diameter.html
-https://www.myg37.com/forums/intake-and-exhaust/220501-coupe-vs-sedan-exhaust-diameter.html

Since I'm in CA, changing or modifying the Cats is completely out of the question, so I need to consider the options from the Cats back. The y-pipe seems appropriately matched to the Cats, so I question if the midpipe is the right place to start as it seems Infiniti made the tradeoffs later in the exhaust with the smaller diameter tubing, which would limit the 'volume' of the exhaust. If you look under the car, the piping after the resonators seems unusually small.

Per the thread I linked to earlier from Tony, increasing the diameter of the exhaust tubing after the collection point within the Y-pipe seems to have little to no negative effects on performance, yet this will increase the volume of the exhaust, which is probably why Infiniti took this path.

If it's truly 2 3/8" at end of y-pipe, then a transition from that point back to a full 2.5" inch exhaust may yield some benefits.

The differences in diameter between stock and aftermarket are pretty easy to see in this thread as the user did an excellent job of taking the pics: https://www.myg37.com/forums/g37-sed...ion-guide.html

 

Fast intentions makes a catback for the X. the piping is bent differently to clear the transfer case.

 

I'm about to buy a used Invidia catback and I'll have my exhaust shop modify it to fit. Seems like the most logical choice and best bang for the buck.

 

Should be a good buy, those are quality sets.