I can tell you that stuff you think its secure with a 'tie down' is going to go flying. IF you look at some vehicles made for high speed driving, many of the parts have small tethers for the hood and truck, etc.
I think the best thing would be to use several brackets -- in addition to the mounts provided -- to prevent the side to side shifting of the rack on the OEM cross bars...the would keep it from wearing on the side rails.