Regarding the stone ceiling in a gas oven...
Is this a gas oven oven without a broiler in the main compartment? If that's the case, then yes, you do want to build a false ceiling in the bottom of the oven, but, it's purpose is more than just radiating stored heat down on to the pie. A false ceiling in a gas oven setup serves three purposes, it radiates heat downward, it takes the heat coming up from the bottom burner and redirects it onto the top of the pie AND it isolates the bottom of the oven, preventing some heat from reaching the thermostat and turning the oven off at the dial temp.
The radiation aspect is pretty simple. Dark color radiates heat more effectively, so build your ceiling out of dark quarry tiles. Also, distance is a critical part of radiative impact- you want the ceiling as close as possible to the hearth, while still allowing yourself enough space to comfortable launch a pizza- 3" is ideal, but if 4" is more comfortable, go with that.
The convective aspect requires a few considerations. First of all, in order for the ceiling to catch the heat coming from below and bounce it back onto the pie, the ceiling has to be larger than the hearth. You want to fill the shelf with quarry tiles and then fill in any gaps with aluminum foil. Another consideration is that in order for the ceiling to bounce the heat, the bottom burner has to be going full blast during the bake. When you've got a burner going full blast, this can complicate things regarding hearth temps. Most materials won't transfer that blazing heat from the bottom of the hearth to the top during a typical bake time, but something like steel, especially if it's a bit on the thin side (3/8"), might. Even if the steel doesn't transfer that blast of bottom energy during the bake, it will transfer it by the time you put the next pie in, so you'll most likely have to wait until the hearth cools a bit. Ideally, any bottom heat scenario (broilerless gas ovens or grills) Will benefit by 3 tiers of materials- a deflector, a hearth above that and a ceiling above the hearth. Certain hearth materials, such as firebrick, have a built in deflection ability, but, in order to bake pizza effectively, they require higher pre-heat temps, and, because of the isolation aspect (see below), you will be driving your temp above the peak oven temp, but you really don't won't to push it that high. In other words, steel doesn't work well without a deflector and firebrick is such a poor conductor that you'd have to push the oven too far to get a good bake. There's two 'happy mediums' between firebrick and steel- cordierite and soapstone. Cordierite's advantage is that it's relatively low conductivity means that it will absorb very little heat during the bake and thus require no deflection. Another advantage is that the lower conductivity will mandate a higher stone temp for quick bakes (700ish), and, the higher you can pre-heat the hearth, the higher you can pre-heat the ceiling, since the ceiling, in this type of setting, is almost always a few degrees less than the hearth. But that higher heat is a bit of a disadvantage as well, as it's pushing the lower half of your oven 150 degrees higher than it's supposed to be going. Soapstone's conductivity will allow for cooler pre-heats, but the ceiling will suffer and soapstone can get costly and hard to find.
When all is said and done, in this type of gas oven scenario, having a 1" cordierite hearth preheated to 700 will be the happiest medium of all. Most ovens, be they electrical or gas, cycle as much as 50 deg. above and below the dial temp, so, with that in mind, 700 is only pushing the oven another 100 deg. beyond that. Cordierite @ 700 might not give you 'pure' Neapolitan bake times, but it will trim a considerable amount of time from where you're at now.
One last consideration with the convective aspect is the placement in the oven. In order to get the most impact from the bottom burner, you want the ceiling as low as possible, but... you don't want the hearth on the oven floor, as it will conduct heat too quickly- so, bottom shelf, hearth, next shelf above that (or closest to 3"),ceiling.
The isolation aspect depends on three considerations. As I mentioned before, the entire false ceiling shelf has to be pretty well sealed with foil. Secondly, because the thermostat is being removed from the equation, there's no way of determining hearth/ceiling temps other than an IR thermometer. Third, although foil won't seal in all the heat, and, if left on, the thermostat would eventually turn the oven off, this isn't something to walk away from. You want to be monitoring temps closely, and, when the cordierite hearth is 700, then launch the pie.
