There is a need to clear this up before it becomes "Internet law".
The graph the Michael showed, has the two curves for the two Mass Air Flow units, they are very similar, slight different at full voltage readings.
The voltage rises as the air flow increases, the ECU uses this to set the fuel and the spark settings, when the voltage signal reaches maximum reading, the ECU goes to the full throttle settings that apply max fuel and spark setting, for a fully loaded engine.
The point that people are missing is that the max. voltage reading for the two units are at different air flows. The 3.3 unit reaches full voltage when it is passing 3.3 lts of air. the Legacy reaches max voltage when it passes 2.2lts. of air. So when you fit the Legacy unit to the SVX it goes into full fuel and spark settings when 2.2lts is passing, it then holds this same voltage setting while the last 1.1lts is passed.
So it won't run lean, it will actually run the same full throttle settings as the SVX unit does, only doing it 33% earlier.
Of course this will only happen at full throttle, when in open loop, at normal driving in closed loop, the Svx will use the O2 sensors to use the same settings as it does with its own MAF fitted.