IMO, soapstone's thermal conductivity (6.4 W/mK) is way too high for the floor of a WFO if you plan to bake at Neapolitan temps or anything close to that. You will scorch your bottoms instantly. I think the right number is in the range of 0.3 - 0.6 W/mK. Certainly not more than 1.1 W/mK.
If you never plan to bake over 600F, maybe, but why mess with a WFO if that is the case?