No matter what an engine is simply a large airpump. A more efficient air pump at WOT, is also a more efficient air pump at part throttle (assuming your tuner tunes at part throttle, yes we do). Hence, if you make more WOT HP/TQ post tune, your fuel economy at part throttle will also increase. We see it happen all the time on a spread of vehicles from Lambo, MB/AMG, to SRT8/10s.
The factory powertrain engineers are restricted by many things including NVH, emissions, beancounters, legal, insurance, etc. We aren't impeded by the same regulations and have access to 100% the same maps the factory tuners do. Also all my tuners are former factory trained apprentice tuners for Bosch, Siemens or Denso. Yes BMW knows what they're doing, but so do we, or else we wouldn't bother.
For those concerned about warranty, we calculate our own checksums (never have CELs) and BMW dealers do not have the equipment necessary to see the changes we make to your ECU file. So warranty is a non issue--additionally we sell our BMW, Porsche, MB/AMG, Audi tuning dealer direct via select dealers worldwide. You don't last very long if you blow up even 1 motor in this game
There's not nearly as much to squeeze out of a S65/M3 cal as there is a M156/C63 however there are still good gains to be had with engine tuning. You will feel the difference at both part and WOT no question.
If there are any specific questions about tuning in general or tuning the S65 in general please let me know.