Pat, you probably already know this, but don't pay huge bucks to get replacement stones from Baker's Pride. Their markup on cordierite is reprehensible.
With an oven that goes to 680 f, and, from what I can tell only has one thermostat, steel is not your answer. In a single thermostat scenario, top and bottom heat are pretty much proportional, so you don't want to use a material that will reduce the bottom baking time. You could, in theory, give the top heat a boost by building some sort of frame to lift the steel decks closer to the ceiling element, but, since you're relatively happy with your current baking times, I think that would be more trouble than it's worth.
The first thing you want to do is measure your existing stones. The literature says the decks are 20.75" x 20.75" with a height of 3.25, but I'm certain that the actual stones are not that thick. Once you get the exact stone thickness, start calling around to local ceramic suppliers to see if they have something close in a cordierite kiln shelf. That will be the closest match to what you have now at the most sensible price.
A single thermostat doesn't give you many options.
Basically, if the top of the pizza is baking too quickly in your target time frame, you'll want a more conductive and/or thicker stone. Otherwise, if the bottom is baking too quickly, you'll want a less conductive and/or thinner stone and/or higher positioning of the stone in the oven.
Does the current stone sit on a flat metal shelf or does it sit on supports? If it's on a complete shelf, then sure, you can play around with quarry tiles. With the lower conductivity of the tiles, you're really going to have to bump up the thermostat, which, in turn will greatly favor top browning, but that might not be a bad thing.
