By itself, a mass air sensor will not increase the performance of a vehicle without other substantial changes to your engines volumetric efficiency. The MAS function is to accurately determine the amount of air going into the engine so that the EMU can deliver the appropriate amount of fuel.
If you change your MAS and the reading it sends to the computer causes the engine to be fed a lean A/F mixture, then you might see a few HP increase. Eventually the EMU figures this out via the O2 sensor readings and adjusts the A/F to proper levels. But, do you really want your engine running lean in order to gain a few HP over the short term?
The stock EMU is more than adequate for any power level you may achieve with your motor, even if you were to open up the exhaust and intake or even change your cams.
At best, it will be a waste of money. At worst, should you melt a piston, expect Ford to deny warranty coverage.