Breadman, I think you've got a winner there. It's really tricky- you need a stone that's conductive, but not too conductive and not too thick or not too thin. At 1-1.5Wm/K, it might be a tiny bit on the weak side, but, as I said earlier, it's better to have to mod the oven a bit higher and gain some more top browning than to have a stone with too much conductivity and/or mass and have to go with a lower temp.
With that conductivity, you might need as high as 650 to get the browning you're looking for on the hearth, but I think that's still a comfortable mod, and, with your sensor, not that difficult to achieve.
I say go for it.