I've heard this idea go around for years, and I've never understood the logic behind it. In my opinion, all a stone placed above a pizza really does is impede the flow of the oven's ambient air, which is already a very poor conductor of heat. Also, while baking stones (and steels) are good at holding heat, neither are particularly good at radiating it, from what I've heard. All in all, using your broiler element at the end of the bake makes a lot more sense than trying to use a stone above the pizza, as far as I'm concerned. That's what it's there for.