In the land of the unmodified home oven, conductivity is king. When compared to options such as fibrament, quarry tiles and firebrick, pure cordierite stones, at about 3.0 W/m-K
, are vastly superior. As you add other ingredients, though, that conductivity increases. Mullite ranges from 4 to 6 W/m-K, depending on the temperature it's at, and corundum (aka, aluminum oxide/alumina) is somewhere in the 18 to 30 W/m-K realm. Both of these ingredients will push the stone above cordierite's realm of conductivity- and that's good. Very good. Greater conductivity = faster preheat = shorter baking times at lower temps. A cordierite/mullite/corundum stone will be better than a pure cordierite one.
Matt, I'm not sure why your hybrid stone performs worse than your 95% cordierite one. The increased conductivity and the greater thickness should give it a substantial edge. The only problem I could see you having is scorching/burning on the bottom when the same temperature/time is used.
Mike, in theory, the CeramicsSF stones could be <1% corundum, but I'm still getting a really good vibe from them. At least, I'm amped about the 1" thick ones. If you can reliably hit 600, then it's possible 5/8" might cut the mustard, but I've worked with enough 5/8" cordierite stones to want to avoid them like the plague. It's just such a small amount of mass.
As far as the pre-heat goes... it really boils down to the quantity of the highly conductive corundum these stones contain. My gut feeling is that a 1" cordierite/mullite/corundum stone probably won't take much longer to hit 600 than your stone does now (maybe an extra 15 minutes), but... because of the extra conductivity, you won't have to bake at 600. With a preheat to 575 (or less), the pre-heat time will shrink, so at the end of the day, the CeramicsSF stone may not take longer to pre-heat than your current one.
I've cut a few stones in my life, so I understand how big of deal it is for me to recommend it, but I sincerely think your ideal scenario is the 18 x 18 x 1 cordierite/mullite/corundum CeramicsSF stone cut to 17.5 x 18 x 1 so the door just closes.