The stock narrow band O2 sensors measure to stoich, but that is not the performance ideal for the engine, which is where the tuning comes in. Honda tuned the NSX ECU to meet specific fuel efficiency and emissions targets. They also tuned it to be reliable in all driving conditions, weather and fuel grades. All of this means something other than 14.7:1, but it is not ideal for power. Most naturally-aspirated tuners assert that a ratio between 13.5 and 13.9 yields max power and a safe AFR. This is their secret sauce though, so you won't find out what it is. But, your narrow-band O2 is really a reference point for the ECU, since even Honda tuned it to a different AFR.
Most tuners concede there is about 20 whp available in the stock ECU through a good tune, but the cost/benefit is high and most opt instead for a supercharger or standalone system. Chip-by-mail won't cut it- you need your car tuned on a load-bearing dyno so each load cell can be tuned to that magic AFR. That's why we're working on getting Prospeed out here.