Your understanding is correct. All dynos I have seen for NA2 headers on NA1 engines show a 14 to 20 whp gain, depending on the car, dyno, weather, etc. This is usually with a stock exhaust. Aftermarket headers will gain more, but not much, and mostly above 6000 rpm. As for the NA2 update, NA1 was 276 hp and NA2 was 290. 290-276 = 14.
![Wink ;) ;)](https://cdn.jsdelivr.net/joypixels/assets/8.0/png/unicode/64/1f609.png)
If you want your NSX to "breathe better," headers are the best bang for the buck. A good set of NA2 headers usually goes for $300 to $500 in the marketplace. Occasionally, you can find a nice used set of Comptech headers for $800 to $1000, if that extra 5 to 10 whp is worth the money to you. The Comptech fitment is perfect and they are the best aftermarket option aside from the super-high dollar baller JDM kits like Fujitsubo and GT-One.
Exhaust is somewhat of a more controversial topic, but I like to think of it as a sound preference rather than a performance adder. People have put the stock mufflers on a flow bench and yes, there is a measureable drop in vacuum, which indicates a restriction. My issue is the assumption that the restriction is a flaw. Honda spent a lot of time and money on the exhaust design, which uses no packing (to save weight) and is frequency tuned to completely eliminate drone. Because the car is intended to be a daily driven street car, you want to have some backpressure because it improves throttle response off the line. Honda knew this. It all boils down to the design goal for the car. If Honda were designing a race car, where the working rpm is 6000 to 8000, they would remove the restriction to free up as much as 5 whp at the top of the rpm range, since the only time you were in the low range is in pit in/pit out. In racing you are going for every tenth on the track, which means chasing every whp. To assume the same design goal for a street car is illogical to me, especially considering the negative consequences like drone and lack of response at low rpm. But, some people want that shiny dyno chart...different strokes I suppose.
Sorry for the long winded explanation, but the bottom line is the factory exhaust is very good. You can get a little more power from an aftermarket unit, but at the expense of drone and a mushy throttle below 3000. Thus, to me it boils down to a sound preference. Many people do not like the factory exhaust because it is too quiet. That is a perfectly good reason to change it. My RM Racing quad-tip sounds BADASS.
![Big grin :D :D](https://cdn.jsdelivr.net/joypixels/assets/8.0/png/unicode/64/1f600.png)
But, it gives my wife a headache. I also think the AP-X, GRF ex-001 and ARC Titanium sound great. The Comptech is decent, too. I think the crazy "F1" pretzel exhausts sound like the fart cans on a teenager's 89 Nissan Sentra, but that's just me.
As for fitment, let's assume you find a set of 97-99 NA2 headers. So, your parts are:
97-99 Headers
96 Muffler
91-94 Cats
Option #1 - Headers --> SOS Adapters --> 91-94 cats --> 96 Muffler
You will need to buy the SOS adapters unless you buy aftermarket headers that come with them. 91-96 mufflers are all the same layout. The difference is in the cats, which in 95-96 changed shape otensibly to accommodate the downstream O2 sensors. This required use of elbow adapters to fit the muffler. I suppose you could use 95-96 cats, but you would then need to buy the exhaust adapters:
Option #2 - Headers --> SOS Adapters --> 95-96 cats --> SOS Exhaust adapters --> 96 Muffler
Finally, you could use 97-99 Cats, which would eliminate the need for the header adapters, but you would still need the exhaust adapters:
Option #3 - Headers --> 97-99 cats --> SOS Exhaust adapters --> 96 Muffler
Your cheapest option is #1.