If you have an airdam that is big then yes the wind would rip it apart. But something that is not so huge should be able to stay onboard.
Passive with rollers is a good idea but would the wind not push it back? How about passive with springs, the wind would push it open.
Is it even worth doing anything to this car?
https://goo.gl/images/DfCjbY
It seems fairly optimised already down there.
At 1:30 you get a view of the underside.
http://youtu.be/nTwkBXN6gtA