The ecu will learn and compensate in closed loop mode/low throttle cruising conditions. The whole key to tuning is to use the piggy back computers to hide the modifications under these conditions. While under open loop mode you use the piggyback computers to adjust fuel ratio and monitor it on a wideband.
Lets say I put 34lb/hr fuel injectors in (100% more flow than stock)and install a SAFC or other maf signal altering device. Before I start the car for the first time after the install I will set the SAFC to cut the maf signal voltage in half(this is completely adjustable in 500 rpm increments across the power range and can be set differently for low and high throttle). The ecu will see half the air and cut the pulse width in half and give the engine the same amount of fuel that it was getting before the injector change.Well, it will be close enough that the ecu will compensate. As long as you are not in the 1-2 milisecond time frame for pulsewidth this will work. Contrary to popular belief, when doing mods like this it is critical to set the car up for low throttle open loop driving before trying to dyno or street tune the closed loop or wide open throttle portion. After all you want the car to run perfectly stoichiometric under light throttle and cruising for gas mileage. After the car has done its fine adjustments through multiple driving sessions you can start tuning the high throttle/boost areas where the car is open loop mode.
Cutting the maf signal in half has about the same effect as starting your car at high altitudes such as 10,000 ft above sea level.
Another method of lowering the amount of fuel is replacing the stock regulator with a rising rate regulator that has a lower initial fuel pressure than the factory 53-61 psi.