While November's figures are very useful, in the context of that thread, for showing how, in most applications, cordierite is far superior to Fibrament, the conductivity quoted for cordierite doesn't tell the whole story. Cordierite varies greatly from manufacturer to manufacturer. It can be extruded or it can be pressed. The formula is rarely the same. The ingredients vary, the density varies, and, most, importantly, it can vary in conductivity. From the research that I've done, 3 is towards the upper end of the conductivity spectrum, with some cordierite clocking in as low as 1.5. Density can tell part of the story (higher density usually translates into greater conductivity), but the formula can impact conductivity as well. The greater the alumina, the greater the conductivity.
Firebrick suffers from similar variations, although the parameters are a bit tighter. Here's an example of a range of firebricks based on alumina content:http://www.traditionaloven.com/articles/84/firebricks-heavy-dense-fire-clay-bricks
As you can see, the conductive ranges from .98 to 1.13. The article references '18% alumina' bricks but doesn't go into technical specs for those, so .98 is most likely not the lowest parameter. I wouldn't be at all surprised if someone was making non insulating firebricks with conductivity as low as .75.
Specific brands of retail cordierite stones probably (hopefully) have static specifications that are easily graphed/analyzed, but, since we don't recommend retail cordierite stones (only kiln shelves), I'm not sure how useful that information would be.
If cordierite and firebricks were single entities with consistent formulas/properties, then this kind of analysis would be useful. The way it is now, though, is that there are too many variables.