There are a whole load of criteria that a manufacturer has to take into account, for different reasons, when mapping the ECU: Driveability and reliability (customer satisfaction), acceleration & top speed (marketing), emmissions and economy (taxation, marketing and legislation).
When you remap, you inevitably compromise one or more of the above, but you may also gain in some areas as well.
Fuel economy is measured on a combination of urban and extra-urban cycles, where the car never exceeds 100 kph, and full throttle acceleration is rarely used. As we all know, this is not how we drive! (Especially with a Porsche). So, remapping to improve engine breathing at full throttle at say 4,500 rpm will not be reflected in the official test (would probably make it worse at low revs), but will mean the car is more efficient at speed and so you may well see an improvement in your actual mpg!
I remember 20-odd years ago, BMW did some tests to find the most fuel efficient way to drive and it came out as full throttle acceleration to max revs in each gear until the required speed was reached and then put it in top gear and stay at a steady speed! Also, brake as late and as hard as possible!
However, this was before the days of variable valve timing, electronic ignition, fuel injection, and fuel cut-off on over-run :wink: