Dave, I'm sorry to hear about the extended preheat. That's a bummer. Btw, that 75 degrees is kind of a best guess. The improved conductivity might buy you a hundred degree differential, and possibly even more.
My local distributor sells 18 x 18 (and 12 x 12) half inch soapstone tiles in the sub $20 range. I know of no one that's baking with them, though. I tend to discourage people from buying them due to potential recovery issues, but with a grill that might not be a problem.
There's also this:https://www.amazon.com/dp/B0000E2OAP/?tag=pmak-20
They talk about a 1" thickness, but that's with the carrier. I'm pretty sure this is 1/2" thick.
We've talked about this elsewhere, but, as much as I'm head over heals in love with soapstones for home oven use, I'm not entirely certainly how well they'll incorporate into a high heat grilling scenario. It is an amazing material, so maybe you'll find an aspect about it that makes it wonderful for the grill, but running through scenarios in my head, I'm just not seeing that huge of a benefit of greater conductivity.
Edit: I think soapstone is the most durable choice for a direct flame grilling scenario, but I think most people are getting plenty of wear out of their firebricks and cordierite.