SB, pretty much all cast iron (frying pan or pizza pan) is 1/8" thick. That's very little thermal mass. You can pre-heat skillets on the stove top to much higher than oven temps, but, while a hot frying pan can be moved around with oven mitts, I'm not all that comfortable moving around a hot cast iron pizza stone, even if it does have handles. Beyond the safety issue, these kind of stove top pre-heats seem to produce pretty inconsistent results. A stone that just works with a fully pre-heated oven, is, imo, far more elegant, forgiving, simple and consistent then any kind of cast iron tomfoolery.
Long story short, unless you're willing to mod the oven to higher temperatures, ceramic stones are generally superior to cast iron cookware.
Right now, as far as I can tell, you're making something very close to Pizza Port. I also think this current oven setup is more than sufficient for reproducing Papa Johns. If you're perfectly happy with this, then I'd suggest a kiln shelf such as this:http://www.axner.com/cordierite-shelf-18x18x1square.aspx
If you live near a ceramic supplier, I'd try to purchase a shelf locally, as shipping can get expensive. Just make sure that it's at least 3/4" thick to match the stone you're working with now.
If you think you might ever want to take your pizzas in a more pizzahacker-ish direction, though, then you want a material that will give you the shortest possible bake times. Right now, the reigning champ is 1/2" steel. It's really heavy, but it will take you down to 2.5 minutes at 550.