Interesting question.
Unordered thoughts follow. Assuming subject is always properly framed.
On the one hand you would think that if the shutter speed was fast enough to prevent blurring, then frame rate would be irrelevant, assuming global shutter or CCD.
But since global shutter is not the norm, you might at some speed be effectively limited by the speed the sensor can be read at.
However if the SS was too slow, and blurring DID occur then you would effectively lose pixel information; exactly how much seems hard to guess, but if the subject can move more than 2 pixels in the time that the shutter is open you might not be able id the face regardless of the resolution.
Then again, there is some sort of clarifying effect that multiple frames from slightly different perspectives has. Most of us have experienced pulling a single representative 'snapshot' out of what appears to be a decent looking stream, only to find that the still looks way more noisy/blurry/low-res on its own. But when viewed at a decent framerate it seems to get sharper.
So maybe we need a PPF/sec metric?