[thstone]

To explain further what aclark133 is saying...

The big wow factor that comes from chipping a turbo or supercharged car usually comes from increasing the boost; more air + more fuel = lots more power. This is usually acheived via mechanical means and isn't a direct result of modding the chip. The chip mod simply adjusts the mix (increases fuel) for the additional air at the first order of magnitude and fine tunes the mix/timing to prevent detonation and lean conditions at the second order of magnitude to prolong engine life.

The problem with a naturally aspirated engine is that the amount of air remains (aproximately) constant so increasing fuel (at the first order of magnitude) doesn't do much. This only leaves the second order of magnitude (a finer tune of mix/timing) changes available to the tuner resulting in not nearly as large of a power gain (or wow factor).

And yes, while some cars are somewhat "detuned" from the factory (for various reasons), it does not seem that the Boxster engines fall into this category and thus, the gains from a ECU tune alone max out at around 10%.