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3-5mpg? 10-20% improvement is remarkable... particularly if it's running higher boost levels. Has anyone looked at AF logs? Must be running a lot leaner (or the timing is more aggressive).
I'd almost feel better if it was unchanged to slightly worse...
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Well, the way these piggy back tuning designed, it won't cause the engine to run lean.
All it did is to tell the computer that the turbo is boosting under the appointed target. So, the stock ECU keeping the wastegate close and the turbo keep building up the boost, until the piggy back tuning tells the ECU that the turbo boost is on a correct target.
With that says, the engine still provides the Air and Fuel based on the appointed boost target.
For example; you step on the gas to push 8 psi of boost, the piggy back intercepted the signal coming from the sensor to the ECU and alter it -3 psi. So the ECU thinks that the turbo is only boosting 5 psi on 8 psi parameter given to the engine. So, the ECU send signal to engine to produce more boost to reach the 8 psi that is originally requested (or 11 psi in the altered signal). When the actual boost reaches 11 psi, the piggy back send a signal to the stock ECU that the turbo indeed reached 8 psi already.
Those are on the simple TMAP / MAP connected piggy back tuning.
While, on BMS JB4 Stage 2, it can alter more sensors than just the TMAP and MAP sensors.