Sorry Norm.. No idea on your FI issue... Does it say how much correction plus or minus its adding? What does the fuel map look like under boost? A NA cars fuel ratio is going to be different than a FI car.. The datalogging does it provide any afr or lambda numbers or is it only the o2 sensor voltages it is giving you? I've burned out a couple of obdII o2's from a rich condition.. Rich=safe for the motor but not good for cats and o2 sensors

I've also noticed poor throttle response and recovery during a rich condition.. Especially if your ecu is trying to correct the short term maps by more than 10 percent.. How much timing does your reflash pull?
The lean condition you mention is it at lower rpms? Or is it under full boost/higher rpms? Your 6 uses the same fuel pump as the 8's right?
Might have to find a local tuner that is good with sc mustangs.... Would be the best $150-200 you could spend

I'll post a question on one of the boards and see if anyone can recommend a local tuner....