i think it's about 7% but it will definatly give better result on high way driving especially when we drive against the direction of air flow.
the paint easily get rough after washing with pressure wash and daily sun abuse.
waxed car will be better in road grip as well becasue the aerodynamics of the car will equally minimize the air friction...especially bonnet. i am not saying that it will result something totally extra ordinary but it do makes difference.