John, it really doesn't matter where the baking element is, as long as it's giving you a stone temp of 600-ish degrees
Steel is heavy. It's the thermal mass that creates a heat sink that provides all the heat (and more) to bake the bottom of the pizza in a faster amount of time. A big part of the New Haven pizza experience is the pizza size. Smaller pizzas don't look as impressive, nor, because of the different cheese/sauce to crust ratio, do they taste exactly the same. For the size that will do a New Haven pie justice (18"), at 1/2" it will weigh in at 45 lb. The shelf should have no problem supporting it, but, it's not easy to remove for baking other things. Since you can hit 600, I'm pretty sure you could probably get away with 3/8", which would drop your weight to a little more manageable 35 lb. No matter what thickness you go with, you definitely want as large a stone as your oven can fit. Any idea of your interior dimensions?
I've been working with another member privately to source lighter, yet still relatively conductive hearth options, and I think I've stumbled on a promising material. It's basically a version of the stone you have now, cordierite, but with a higher alumina content and a dry pressing manufacturing process giving it greater density. Between the density and the alumina, this stone has greater conductivity than the kind of cordierite, you (and most retail oven stone owners) are using. Here's the link for one of the better deals on one of these 'high alumina' stones:http://www.baileypottery.com/kilnfurniture/kilnshelves.htm
The 18 x 18 x 1" stone, at 25 lb. is much more portable than 18 x 18 x 1/2" steel at 45. It most likely, at the same temps, won't give you comparable bake times to steel, but it should definitely hold it's own. Two downsides: One, it's not cheap. The stone is $50 and, by the time you add shipping and handling, you could more than double that. Steel, on the other hand, should be able to be sourced locally for $40. The other downside is that, so far, no one has purchased these exact stones, so, even though I'm 100% certain that they'll give you a bump in conductivity, how much of a bump is a question mark.
Member Essen1 (Mike) purchased a high alumina stone (also culled 'mullite'), but he went with 5/8" and, though he was very happy with, it didn't have quite enough thermal mass to give him fast bakes.
It's up to you, $100+ for a 25 lb. high alumina cordierite kiln shelf vs. $40ish for locally sourced 35 lb. 3/8" steel plate. Assuming you can reliably reach 600 degree pre-heats, both will give you the full range of Pepe bake times - 6 to 8 minutes without any oven tricks or modifications.
Regarding porosity... porosity translates into air, and air is bad for baking stones on two fronts:
1. Where you have air, you have lack of density, which, in turn, usually translates into lack of conductivity. Besides our need for conductive materials for faster bakes, conductivity is critical to the durability of the stone, because, as the heat travels faster through the stone, the lack of hot and cold spots prevents drastically different rates of thermal expansion, which, in turn, prevents stress
2. Where you have air, you have the potential to absorb water. Water + heat = steam = massively expanding volume. Steam puts an incredible amount of stress on a stone.
Even if your stone is a particular material that is both porous, conductive, and durable, for the temps that we make pizza at, water on the bottom of the crust doesn't stay water for long. Once it turns into steam, it's on it's way up, not down into the stone.
That's great regarding your willingness to track down/work with Full Strength flour. The difference between blended Bouncer + AP and Full Strength won't be dramatic, but every little bit helps. Where I am in NJ bromated 14% (All Trumps, Kyrol, Bouncer, Balancer, etc.) has the market share, so bromated 12.5% is difficult to get. I've got to drive an hour to distributor who carries it and is willing to sell to the public. It might be different in New Haven, but I kind of doubt it. I'm not telling you that you won't find Full Strength (or a 12.5% analog), but don't be surprised when you find lots of sources for bromated 14%, but come up short on 12.5%.
Speaking of analogs, bromated 12.5% flours are, in my opinion, relatively interchangeable, so, as you look around, don't look just for Full Strength. There's also
King Midas Special
Perfect Diamond (I think this is 12.5%ish, but not sure)
I should also mention that Gordon Food Services has their house brand of bromated high gluten flour (Primo Gusto), as does Sam's Club. These may be 13% protein, or they may not
They don't publish specs so it's difficult to determine for certain. A lot of members have done very well with these private label flours, so you might want to give them a try, but, at this point, I'd like to see you work with flours with more tangible specs.
Btw, what area of FL are you in? If you can tell me an area, I can help you source some potential distributors.