Has there been any proof that these covers actually lower the rear diff fluid temps?

They're pretty pricey for just looks...

 

JMHO, but I think it's obvious that increased surface area will increase thermal transfer, however, that's the wrong question. I would be asking: Has there been any proof the differential of an F150 being operated on the street needs additional cooling?

 

The limited amount of fins on those coves won't significantly reduce diff temperatures and the thickness of the cast aluminium ones actually significantly reduces heat transfer. Those covers are made for Fan Bozes that value image over function. To me, mostly they say "Hey look at me, I've got too much money and too little judgement and I think I'm Johnny Rocket!"

 

While I pretty much share the overall sentiment, not buying into the idea that the thicker material reduces heat transfer and would love to see it substantiated. If that were the case, thin over bored cylinder walls would cool better than thick stock ones.

 

There's a big difference between cylinders that are only .005" difference in wall thickness verses a thin stamped out steel cover that is probably 1/32" thick and a cast aluminium one that it roughly 3/8" think.

If you want to substantiate that then get a thin steel pot and a heavy cast aluminium one such as those by Club Aluminium and go put each one on a HOT burner on your stove and see which one gets HOT inside the fastest. Thin metal will always conduct heat much faster than thick metal.