Contrary to popular belief the engine is not maxed out for performance. Other factors such as reliability, noise and emissions are taken into account. For example there is evidence to suggest that on the 2L engine the motor will spin to 10000rpm without causing problems. The engine cut-out is a more sedate 9000rpm.
One reason why people change their air intake or exhaust is because of the sound. This is also the reason that they may fail emissions testing (too loud/emissions above limit etc). The stock dual exhaust is heavy compared to an aftermarket single exhaust. The downside of the aftermarket exhaust is that in many places it exceeds the local noise pollution limits (eg above 91 dB).
There have been various revisions to the S2000 over the years. The suspension geometry of the '07 and the '00 are different, the wheel sizes are different. The engine displacement has been increased and gearing revised. Even the air/fuel ratio between the different years of the AP1's has changed.
The drag racing comparisons I do not feel do justice to any mods. It does not refer to the baseline of that particular car, and furthermore the most telling factor in drag racing is the driver (shifting points, shifting speed, take-off). Similarly at the track driver skill plays a big part, unless of course you are side by side going down the straight.
Personally - I'm leaving my car stock (unless of course you count changing the oil, fuel and tires as a mod). But then again I have a perfect car with no clutch delay valve or intrusive VSA
