You certainly need a lot of BTUs below, and it's really helpful if they are spread out a little so you can adjust down the one directly below if needed. The ceiling height an be mitigated with insulation as I demonstrated in my grill mod: http://www.pizzamaking.com/forum/index.php/topic,9614.msg83372.html#msg83372
To get into NP times, I would think this box would be much better served with some insulation up top. Heat shoots straight through that thin metal.
I ran the radiant burner when baking pies, but it didn't make much of a difference. Most of the IR from the burner shot straight over the top of the pie. Adequate insulation was the deal-maker or breaker. The Fibrament grill stone I used had a) fairly low conductivity, and b) a metal pan that would deflect at least some IR and create a tiny air-gap between the pan and stone. This helped immensely in managing bottom heat. Before the Fibrament, I was using a cheap stone, and without some foil below the stone, I almost couldn't run the center burner.
I'm a bit dubious how this box would perform on my grill for NPish pies. There is no deflector I see on the schematic, so I would need to heavily rely on the two side burners, yet the oven isn't that wide. I think foil would be required to direct convective heat into the box. The top and sides of the box are so much more conductive than the stone center panels, the vast majority of the heat is going to exit away from the baking chamber.
I don't buy the 800F claim from a practical standpoint. Perhaps if everything in the set-up is idea, but not on the average grill. Can it hit 800F on the bottom, probably, but I don't see it being balanced, and it's probably well under 800F above the pies when 800F below. It's tough to get the air above that hot. It literally took me 10 layers of the insulation I was using to balance the heat. I bet you would need at least 1/2" of good ceramic insulation in exterior walls of this oven. Maybe I'm wrong. Maybe having this box inside a closed grill is enough?