then I think it could be the ecu going into closed loop mode. I don't know much about your ecu, but most ecu's after a curtain throttle will switch from open loop mode to closed loop mode,
Open loop, takes information, MAF, IAT, AFR, and adjust the map accordingly
Closed loop, follows the stock ECU map without regard to any readings
this would make perfect sense if you went pig rich, what is stock 240? to 440? an 80% increase, with stock map, that would be pig rich. but to go lean. maybe the corrected MAF settings are messing with the ECU when it enters closed loop mod causing a lean condition.
this is one of the reasons why I chose to go with megasquirt in my ford probe it had this closed loop mode and it didn't care about any MAF readings.
when I first installed my supercharger kit, I didn't do anything for fuel, becuase the MAF does not understand pressure, it understands airflow, so with the supercharger its just more airflow, so it would add more fuel, to a point. 4 psi seemed fine. another ford probe guy did this with a turbo at 5psi.