That's okay Tory, I knew you were talking to me
The new 911s are 500 deg. peak temp stock, but have a 650 deg. option (911p). I'm guessing that they probably changed the configuration over the years. With a 650 degree peak temp, there's some hope.
Let me explain the paint issue another way. Shiny metal = reflected heat = better heat retention. Painted surface = absorbed heat = worse heat retention. I don't think this is going to make a huge difference, but I think it's something to be aware of, especially if the exterior of the oven starts getting hot.
Generally speaking, deck ovens have thick enough stone decks to retain heat well when the door is opened.
Getting Neapolitan bake times out of 650 deg. gas ovens has been discussed recently:http://www.pizzamaking.com/forum/index.php/topic,14339.0.html
Installing a brick ceiling will help, but I think it will only get you so far. A traditional Neapolitan oven has a vertical space in the 13" range, but the dome is 1000+ degrees and the (usually) lit fire on the side is throwing off a lot of radiative heat as well. A firebrick ceiling at 650 deg. is pretty anemic. And, unlike the greater conductivity of a steel hearth, steel doesn't radiate heat any better, so a steel ceiling will be equally as anemic.
I've never seen anyone do this, so it's strictly theoretical, but you might buy yourself some top heat IF you put in a brick ceiling AND you raise the hearth to within 3" of that ceiling. You couldn't put any pizzas in the back of the oven, so you'd be limited to two 14" pizzas side by side at a time. Before you do this, though, I'd test it in a home oven with a cleaning cycle hack.
If you can
muster up enough top heat for Neapolitan, then choosing the right hearth material might involve some trial and error. You need every amount of top heat that you can get, so you have to run the oven at 650 and no lower. It's tricky. I think 1/2" or 3/4" steel at 650 should give you Neapolitan bake times on the undercrust, but I can't say what the recovery time is going be like. If you go thicker, then the recovery improves, but you also burn the bottom before the top is done. Cordierite's out of the question as is soapstone. In a perfect world, there would be a hearth material with a conductivity somewhere between soapstone (6 w/m-k) and steel (30ish).
Are you sure you don't want to sell something more along the lines of an Apizza Scholls pizza? It would make your life easier. You'd probably still need a custom bracketed brick ceiling, but it would guarantee you Apizza Scholls bake times without too much experimentation. I'm not saying that Neapolitan is impossible in a deck oven, but I do know that no one has done it before. The closest anyone has come is Jeff Varasano, who had a Swedish oven manufacturer custom build him an electric deck oven capable of reaching 800+ degrees, and, although I think he has the capability of Neapolitan bakes, he's still only using if for coal-Neapolitan hybrids.