Or is there some property of steel that makes 520 or 540 behave more like a 600-degree stone?
Ryan, it's called conductivity
Stone transfers heat at a slower rate than steel. 525 on 1/2" steel produces identical results to 600 on stone.
That's the beauty of steel. It can make a 525 deg. oven act like a 600 deg. one. Since most people don't have ovens that can reach 600, this is what makes it so revolutionary. The only catch is that, in order to have a balanced bake, you have to use the broiler to bolster top heat during the bake, which means you have to have a broiler in the main oven compartment.
If you can comfortably hit 525, though, and have a broiler in the main compartment, you can make these pies in your home oven with 1/2" steel plate.
Regarding steel's heaviness... that information is a bit dated. As a single chunk of 40 lb. metal, steel is pretty brutal to have to deal with. But members have been cutting the steel down the middle and baking on the two pieces. 20 lb. pieces are much easier to work with.