Norma, generally speaking, cordierite (baking stone/kiln shelves, etc.) is really not that different from firebrick.
Here's how I see baking stones. You've got your chemical bonded cementious stones (Fibrament, cheap Walmart stones, etc) and your heat bonded/sintered/kiln fired ceramic stones (fire brick, cordierite, schamotte, rokite, etc.). For a while, based on the fragility, I thought the BS stones could be cementious (cementious stones are notoriously fragile), but, now that I've seen the BS stones in person, I'm confident that they're ceramic. When you get into the ceramic realm, the manufacturing processes vary, as do the ratio of ingredients, but the core ingredients tend to be the same from stone to stone. Every brand of baking stone/kiln shelf, just like every brand of fire brick, is going to have slightly different thermal properties, but it's all pretty much in the same ballpark- at least, it's similar enough in a blackstone setting, with it's innate ability to control top and bottom heat a bit more independently than a wood fired oven, that the differing properties aren't consequential.
The three very slightly different materials you're looking at might give you slightly different conductivities, but your issue is not related to conductivity. The contrast that you're trying to avoid is a result of an unevenly heated stone (and possibly a stone that's a bit too hot). Fire brick isn't going to help with this, nor is the other cordierite stone your experimenting with. If the system is giving you hot spots on the stone, they'll give you the same hot spots on fire brick and on a different cordierite formula.
The BS tends to heat the floor a bit unevenly. We're seeing this with the char rings some members are encountering along with the contrast you're trying to avoid. As of this momement, on the Neapolitan side, we don't really have an easy solution, but, for NY, achieving an evenly heated stone should be considerably easier.
The first thing I'd do is to lower the pre-heat temp. Higher temps encourage contrast and inhibit crispiness. Try 25 degrees less, maybe even 50. As you bake with this, carefully check the temps on the stone, from the center to the outside, and make sure you're not seeing a huge disparity.
If you do see fluctuating temps on different areas of the stone, I would try a different tack. It was Neapolitan, but the most even undercrust I've seen on a pizza yet with the BS was a pizza Chau made, and he happened to bake it AFTER he baked something else. If your stone is unevenly heated, if you turn off the heat, an equilibrium will eventually be reached. Try taking the stone 50 degrees higher than where you want and then give it enough of a rest to cool by those same 50 degrees. By the time the temp has dropped, the stone should be all one temp and should produce a very even bake. We are talking about an extra investment in time- maybe 30 minutes to reach the peak temp and another 30 minutes to cool down/even out. But it should help.
One other thing to consider regarding lower pre-heat temps is that the dial on the BS is calibrated to provide either massive heat (when cranked) or slightly less than massive heat (at the lowest position)- both are generally a bit too much for NY, especially the crispier sub style you're striving for. In order to tame the flame, you've got to carefully dial down the knob on the regulator a bit.