That's the point. You lower the rad to keep it matched to the engine fan.
The valence forces airstream under the truck which siphons hot air out from the engine area by venturi effect at highway speeds.
This gets tricky.. guys on the TBird site have noted what happens to cooling at speed when they dont replace the air-dam which commonly gets ripped away by parking curbs. Putting the air dam back on increases radiator efficiency at speed because the venturi effect isnt enough.. need to create a real vacuum under the car.
Check out NASCAR front ends, not that low just for high speed traction.

 

Ohh, I didn't know that. I don't drive the highway much, actually yesterday was the first highway driving my blazer saw.

 

I have driven 10k miles with mine removed and have seen zero difference in engine temp. I do allot off hwy and city with heavy traffic driving and there has been no difference. The plastic valence won't protect anything offroad either. I suppose if you don't have skids it may help protect the oil filter from a stray rock or something on the hwy. I personally just don't think it does anything but add some cosmetic difference either good or bad depending on your personal opinion.

 

Agreed