In a frontal impact, only the front airbag will deploy and the Garmin would be moving forward and not backwards towards the driver, unless it ricochets off the windshield and comes directly back at the driver. Even then, in order for it to hit the head of the driver it would need to get past the steering wheel, and the deflated airbag. However, the relative height of the Garmin is closer to my chest than head. Regardless, having the Garmin mounted near the rear view mirror may actually pose a greater threat to one's head. For that matter, the V1 and the Broadway mirror in the center might be bigger issues than the Garmin mount.
In a side impact from the driver's side, the Garmin will be sent towards the passenger side of the vehicle and not directly back at the driver's head. In a side impact from the passenger side, the Garmin unit has no where else to go. If I had the Garmin mounted on the rear hatch glass directly behind the driver, it may pose a projectile danger, however even in that situation the rear seat back is high enough to block the Garmin brick from hitting your head.
In relative terms, I think I would be more worried about the chrome bumper from a lifted F150 hitting the side of the STI than the Garmin in a side impact.