I installed the GM Parts Rear Air Deflector and wasn't satisfied with the way it looked so I cut 3/4" off the stanchions that mount it to the body and used different hardware so it is a strong mount. If I was to do it again I would do it exactly the same but mount it 1/2" forward on the body so it would disappear a little more.
I got this to cut down on the dust all over the back. You would need to have vents made into an extension to do that. I like the idea of the extension being part of the body so it follows the lines.