Best power, fitment, and affordability: Nismo "take off" exhaust, w H-pipe
For more power add any of the following:
1. ebay or other non-resonated test pipes - cheapest, but also most raspy and not very eco friendly...
2. Any resonated test pipe - more expensive, much less raspy, and as above.
3. The infamous ART pipes - much more expensive, sounds good, a bit more power, and as above.
4. Various High Flow Cats - slighly less power, varying degrees of rasp depending, and more eco nicey nice...
5. Various long tube headers (my personal recommendation = PPE). Much more power, good sound and modest rasp (varies), and there's no way to stop the tides from rising at this point anyway... (or weld in a cat somewhere if you wish/must).
Moral of the story: The OEM cats are the biggest restriction...
Also, changing the size of the OEM Y pipe pipe (not sure on the Nismo H if the diameter is changed from OEM...) generally results in a lot of additional "whooshing" noise on decel.
Good luck
