Dust shields are almost 100% useless. I don't run them on any of my cars anymore and haven't for tens of thousands of miles. Lots of mud might be the only exception, and I'm not totally convinced on that either but I don't run any of my stuff in deep mud often enough to know myself. All of my cars get driven on gravel roads. Dust shields are useless on gravel. The road dust gets through, the brake dust doesn't get out, the heat is held close to the rotors. And as an added bonus, every now and then a rock gets trapped between the shield and the rotor. That's lots of fun.
Brake ducts are pretty much a race only deal. As already pointed out they can actually be detrimental for street driving, although mostly I figure they just don't do much under normal street conditions. If your brakes are overheating and fading on the street you've got other problems.