Hard to say. Typically the path of a rock that also has the ability to crack a windshield would not be affected by the aerodynamics of the bug guard - pretty much meant to re-direct bugs (with airflow).I am waiting until gravel on the roads season is over.
Do those bug deflector hood thingys help any?
I probably already know the answer!
That said, a direct path rock could be stopped. I am going to guess a 7.2% less chance of rock hit with vs without.
I have one...like the look but can't say it helped...that I know of.