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The airflow near the front tires is rarely travelling straight towards them, so the design of successful tire deflectors may not be intuitive.
High pressure at the bumper may be forcing flow both down and outwards, which is one reason why OEM spats don't extend out to the edge of the tires (they'd be overkill if they did, pushing air further out than necessary).
Some OEM spats that are angled similar to your Figure B would be subject to perpendicular flow! (Of course, it depends on the vehicle.)
One Volvo study found that even very small changes in spat size / positioning can have a negative effect (increase drag). I'll see if I can dig it up...
This is a tough one to guess at!
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