Wow, Mark, really nice job. Well done!
Now, as far as the numbers go, I'm seeing some interesting things here. As predicted, the lower compartment is about 100-150 degrees hotter than the top. The 485-545 disparity on the stone- that makes a lot of sense, as it's going to take a long time for heat to travel, for the most part, from one side to the other. Thermal mass, as I said earlier is a disadvantage here.
I know Larry has pretty much put the no gap 1/4" steel plate method to rest, but I'm still holding out a little hope. Foil is going to let a lot more heat pass through it than 1/4" steel will in the same amount of time. An oven's btu/wattage is going to play a part- the more heat the bottom burner can pump out, the greater the disparity, the hotter the plate will get before the thermostat cuts off the burner.
At the end of the day, even if the no gap technique does work, it's most likely only going to work well for one pie- which really doesn't make it all that viable.