[Duk]

Simply adding more fuel doesn't mean more power. A richer air/fuel ratio burns cooler and slower, resulting in lower combustion pressure and less torque. Most computer controlled cars run rich at high loads, and bennefit from leaning out of the air/fuel ratios.
If you can find an after market chip that can actually acheive a power increase on a standard engine, then it will more than likely have a combination of leaner mixtures and some extra ignition advance.