With evasive answers like that, Fibrament should probably run for office
They're throwing around a lot of scientific jargon to make it look like what they're saying has heft, but, in reality, it's just a silly attempt to try to make people believe that every non fibrament stone is garbage. If less conductive stones are inferior for baking that would denigrate every firebrick in every Neapolitan oven across the globe. All ceramic stones are insulators, and, by nature, fairly poor conductors. That's what makes them take forever to get hot and that's why they stay hot for a long time. The range in conductivity between ceramic materials is pretty negligible. Any stone can make phenomenal pizza as long as it has sufficient thermal mass.
Fibrament basically doesn't want their non commercial customers to feel like they're getting something less than spectacular, so they pat them on the head and do a little misdirection about heat transfer. The reality, though, is that the thinner/smaller the stone, the less thermal mass it contains, the less heat it stores, the longer it takes to bake a pizza. It's not rocket science.
I've cooked pizza with 3/8" stones, 1/2" stones and 2" stones, all preheated to 550. As you go larger, the cooking time decreases dramatically. That's why my local pizzeria (with what is most likely a 2" slab) can cook a pizza in 6 minutes or less with an oven preheated to only 450.
Looking at the picture of your stone in the other thread, it looks like it could be either 1/2" or 3/4". A square 18" wide 1" thick stone would shorten your cooking times/improve your oven spring a bit, but a 1 1/2" thick stone would take you into real pizzeria oven thermal mass territory. Both would probably cost you a truckload of money, though.
A cheaper option would be half width firebricks (1 1/8" thick). Most brick places carry them. I've worked with 2" firebricks (that are heavy as lead), but never the half width ones. I've always kind of wondered whether or not the half width bricks would shift during launching, but I'm reasonably certain that won't be an issue.
Presently, I'm getting 3 1/2 minute pies from a 17" x 20" x 1 1/4" soapstone slab that I picked up as a remnant from a local soapstone supplier for $25. I've used just about every possible material for baking stones and put in years of research and I can say, without a doubt, for the home pizza baker, nothing comes close to soapstone. It's the Rolls Royce of baking stones. It's very dense/stores a tremendous amount of heat and is thermally almost indestructible.
Here are two suppliers for Florida:http://www.creativesoapstone.com/contact.htmlhttp://www.doradosoapstone.com/_DS-Florida/index.htm
All soapstone places have remnants. It's just a matter of finding an 18" square. If you find a piece that's slightly larger than you need, maybe they'll cut it for you. I wouldn't recommend trying to cut it yourself, unless you know someone with a tile saw.
If these places are too far away, look in the phonebook under soapstone contractors. There's a good chance one of them will have a scrap piece lying around that they'll sell to you.