You left out a couple important data points: what kind of oven, what kind of pizza, and how thick are the stones?
I can't remember anyone ever talking about a cordierite stone with density that low. Are you sure it's cordierite? Low density may mean lower thermal conductivity, and it will mean lower the heat storage - linearly, AOTBE. I think most of the cordierite stones we see are around 2.6, so even the denser stone you mentioned is almost 40% less dense. That's a lot of heat storage. The only way to get it back is with thickness.