Oyvind, soapstone does heat evenly and it's extremely durable in a direct flame setting, but it may not be the best material in every grill scenario.
The biggest obstacle in grilling is baking the top of the pizza as quickly as the bottom. With heat (generally) coming from below, the hearth is invariably putting out way too much heat as compared to the heat radiating off the ceiling. This anemic ceiling heat can be mitigated, to an extent, by lowering the ceiling as much as possible and by using a deflector beneath the hearth to point hot air away and up, but it isn't an easy task. By using a hearth with a higher conductivity (soapstone), you're baking the bottom of the pizza faster and creating more of a ceiling/hearth ratio problem, not less.
Member Jackie Tran found grill success by essentially handicapping his hearth by using relatively poor thermally conductive firebrick. Firebrick is durable in direct flame settings and it's the cheapest baking stone material you can buy. Depending on your heat source, though, it can heat unevenly. He is working with 700+ degrees and a gas flame, so his situation is a little different.
As far as the best material for baking pizza- it's not necessarily ceramic. It's the material that can transfer the right amount of heat in the correct amount of time. In an electric home oven that can't output the same amount of heat as a commercial deck pizza oven, then you want a conductive stone to make up the difference. In this scenario, soapstone is ideal.
550-600, as far as grills go, is kind of chilly. It's basically an electric oven without the broiler. And that 'without the broiler' factor is bad- really bad. Hearth-wise, thick cordierite (1") or soapstone works beautifully at that temp (for NY style pizza), but the ceiling- without a broiler, you're up the creek without a paddle. I'm in the process of testing a two soapstone setup with charcoal being placed below the hearth AND on top of the ceiling stone. I think, if I go with enough charcoal on top, I might be able to have a decent ceiling to hearth ratio. Maybe. I went with soapstone because I can get it relatively inexpensively, but if you're paying more than $30 sq./ft. for it, then I'd go with what I believe will be a superior ceiling material- 3/8-1/2" steel slab.
At some point, I'll be putting charcoal right on top of my soapstone baking surface, but I'm not that excited about the prospect of having to brush ashes to the side to bake the pizza.
I'm also working with a grill with a cooking area of 19 x 25 with a tall enough ceiling that I can basically build whatever ceiling setup I want. A Smokey Mountain doesn't give you much room to play with.
I'm not saying it can't be done, but you're looking at a difficult task. I'd read every 'egg' thread you can find here as well as Jackie's experiments with charcoal.
