Dave, soapstone, imo, is more forgiving thermally than cordierite. At least, high talc soapstone without too many imperfections- which is what you/most other soapstone owners have. We are talking about two incredibly thermally durable materials, but, I think, in a direct flame setting, soapstone will outlive cordierite.
Before you take the time to look into ways of cutting/grinding the stone you have, I would call Creative and see if they have the 18 x 18 x .5 tiles. I would think that spending about twenty bucks on a tile would be far less headache than cutting a stone in half lengthwise.