Mark, the threads from Peter are a useful read, but it's important to remember that Heston is working with a 1/8" thick cast iron pan and Outatime's pan, although the edge of the base is 1/3", the pizza baking surface, like most cast iron, is 1/8" as well. Rotoflex is also relatively light gauge, relying on the convecting hot air to feed the deck/bake the pizza rather than the heat stored in the steel deck itself. My 1/2" steel plate suggestion is based on the premise that 1/2" will give you the necessary specific heat to store enough energy to cook the entire bottom of pizza. At 16 x 16 x .5, we're talking a beast- about 40 lb. of mass.
It's best to obtain the steel plate locally. 40 lb can really drive up the shipping charges.
To be honest, although iron and steel have some history with pizza, 1/2" plate is an entirely new frontier. It's all theoretical. I stand behind the theory and the math, but, until someone actually buys a plate and starts baking with it, it's just theory. My soapstone slab gives me 4 minute pies at 525, and, with some futzing, 2.5 minute pies at 550 (the broiler keeps shutting off, but if I baby it, I can get good top browning). Since 4 minutes is my ideal bake time, I'm not running out to get steel. Soapstone doesn't really help sub 525 oven owners, though, so I've started recommending steel. Right now, if someone's oven doesn't go above 500, steel is their only hope.
I'm cutting it a little close on the thermal mass, but, because of the weight of the steel, I have no other choice. As I said before 16 x 16 x .5 weighs around 40 lb. As you go up to 3/4" thick, that jumps to 60 lb. As much as I like that much thermal mass, I can't recommend people put a 60 lb. plate on an oven shelf. 1/2" steel has about the same specific heat as 1/2" cordierite. The steel should be far more effective at transferring more of it's heat to the pizza, though. With cordierite, after 4 minutes, the part near the pizza will be much cooler than the outer extremities of the stone. The cordierite will have hot and cold areas. With the increased conductivity of the steel, their will be a greater equilibrium in temperature- and thus more overall transfer of energy.