First off, excellent sound, overall. Given that this is your first sound mod, it really deserves 5 stars, but then my constructive criticism below wouldn't be worth as much :)
Internal : Good balance of intake bellow, exhaust wail, mechanical noise, driveline quivver and deceleration. The quality and choices of your samples is excellent. I can hear some slight sample overlapping around 3/4 of the rev range, but it is only slight. Pitch phasing is mostly smooth and not too apparent, and each layer remains well separated both on headphones and analog 5.1 speakers. The car sounds as aggressive decelerating from 30 mph as it does under full load down barreling down a straight, which can seem odd at times. Idle loop is a little incongruous with the otherwise excellent driving sound. I found myself over-braking just to bang more downshifts.
I would recommend the following as improvements :
1)more mid-high samples to reduce overlapping and pitch phasing effects, creating a more natural transition.
2) Please add road and wind noise. Only a handful of mods feature this, the updated NSX is one of them. The immersion bonus is massive when you can hear the rumble of road, rubber and air around the car.
3) Although already energizing as it is, this mod could use even a little more gain at higher rpm, especially prior to shift points. Maybe even a bit of saturation/distortion into the redline to make things sound crazy before upshifting.
4) Upshift clunks would increase enjoyment and compliment the driveline quivver/lash.
5) A rev limiter that operates on ignition cut vs fuel cut (louder, more aggressive and more habitual on race cars)
External sounds:
Samples are great. Decel is way too silent at all rpms. High-strung NA engines are extremely loud when decompressing, and with 500 bhp from 4.0 liters and no mufflers, this thing should be rasping and clearing it's throat off throttle at high-rpm. Take a listen to NeverKrash's 599xx sound mod as inspiration. The internal downshifts are way too satisfying to not be reflected on the outside.
Overall, very much worth downloading and enjoying! Extremely impressed that this is your first sound mod as it is genuinely good and exciting. I hope my feedback was helpful and keep the updates coming!