This is the most incorrect/misleading thread I have seen in a long time. S-Tune makes more gains then the OEM Nismo?! There seems to be a lot of confusion about what exhaust is what and a lot of people are using many different names for the same thing.
Stock OEM Z, Y-Pipe design, what the base and sport package come with.
2.4" Y pipe
Cross Section Area 3.52 sqin
S-Tune Nismo, Y-Pipe, not found on a Stock Nismo 370Z
2.4" Primary pipes, joining into a single 2.5" y-pipe, then splitting back to two 2" pipes, through two resonators which stay 2" inside then back up to 2.4" out the back and through the muffler.
~5 whp stock OEM exhaust.
Cross Section Area 3.97 sqin
OEM Nismo, "R-Tune", H-Pipe what comes on a Stock Nismo from the dealer.
1.75" Primary pipes that stay true dual throughout the exhaust. They increase to about 2.25"-2.3" later on down the pipe run near the mufflers. There is a H-configuration crossover near the engine.
~10 whp over stock OEM exhaust
Cross Section Area 4.8 sqin
FI Exhaust
2.5" X-pipe
~20 whp over stock OEM exhaust
Cross Section Area 9.8 sqin
IMO the biggest concern is cost a used good condition Nismo exhaust can be had for 500. A FI exhaust will be hard to find used and a new one will set you back over 1,200. For a difference of about 10 whp.