Mass airflow is great for precise airflow measurement -- cruising conditions, everyday driving, fuel economy, etc... But MAF sucks at measuring boosted conditions (sudden rush of airflow through the heated element).
Supras (MKIV) uses both MAF and MAP. The MAP is there to measure boosted conditions and the MAF is for precise airflow -- one of the best systems out there.
By converting to MAP, you don't gain any power on a stock motor. In order for you to gain power, the MAF unit itself must pose as a huge enough airflow restriction.
If your motor is near stock, the MAF will actually provide more precise readings for the ECU.