If you remove the airdam lip and the cover, the car will tend to overheat. I broke the airdam lip while parking and it also ripped the lower cover, so I left them off. The engine temperature started to creep to higher temperatures. Then I remember that many moons ago, when I had my late model Pontiac T/A, the airdam lip also broke and I left it off. The car started to overheat during traffic stops.
I asked a friend that used to work at the GM proving grounds and he told me the airdam and the lip are designed to flow air directly to the radiator area. He also said than when the car sits idling, like in traffic, they also redirect the air the fan are sucking. I replaced the airdam lip on the T/A and the problem went away.