Yes, Ripper is right... And I also dont think a fault hat failed the vehicle from the emission test won't be corrected by ECU resetting. (You haven't mentioned about a emission test fail...) An ECU always try to use a closed loop to control the fuel delivery. Unless the ECU running on safe mode or at several engine situations, such as cold start or full load conditions, the efficiency is maintained at highest level. performance vice it may have a effect....
And another thing... high performance means low economy..... resetting ECU and driving economically will reduce your fuel cost..... but I'm sure that will not be a large measurable figure....