Jordan, your oven is really not that bad. Without modding, it's not going to match the results of a WFO, but, until you build a WFO, you might as well make the most of it.
If you can continue doing what you're doing to achieve that 670 pre-heat, along with possibly moving the stone just a tiny bit closer to the broiler, as well as obtain the right stone, you're going to achieve one of the most Neapolitan-ish bakes ever witnessed in an unmodded oven.
The question I keep asking myself, though, is what's the 'right stone?' Not only is the stone 670, but, since your stone is close to the ceiling, the ceiling is most likely near 670 as well- that's a big part of the success you're achieving- you've got a bright red broiler during the entire bake PLUS you've got a 670 ceiling radiating additional heat down onto the pizza. In order to continue to get good coloration on the top of your pizza with your current bake time, you can't go any lower with the pre-heat temp. Not a degree. So the big question is "what stone can you buy that will give you leoparding on the undercrust in about 2.5 (or slightly less) minutes at 670?" The answer is "I'm not sure."
The problem is that if you get a stone that browns the bottom of the pizza too quickly, you don't have an option to turn down the temperature. If, on the other hand, you buy a stone that doesn't give you enough coloration on the bottom, then modding is your only option. 670 is already pretty hot for an oven. I know that Marlon (Bakeshack) get's authentic Neapolitan bake times by covering his oven probe and pushing the temp to 790 (along with a freakishly powerful broiler), but as the temperature goes up, I get nervous. I don't think any of this is dangerous, but it could be hard on the oven. How much trouble would you be in with other members of your household if you broke the oven?
Salvatore is getting really beautiful undercrust leoparding at 650 with 1.25" soapstone:http://www.pizzamaking.com/forum/index.php/topic,16117.0.html
Soapstone is, imo, too much of a gamble, though. You could end up with a less conductive piece and not get enough color.
Steel is the answer, but picking the right thickness is going to be tricky. 1/2" steel performs similarly to soapstone, although it's a bit more intense. In a year or two, we'll be able to graph different thicknesses of steel and corresponding bake times/temps, but right now, there's no one working at 670 with steel. The happy thickness is going to be either 1/4", 3/8" or 1/2". I know that doesn't narrow it down much, but that's the best I can do at the present moment. If I had to bet on one of these, I'd go with 3/8", but bear in mind, if you overshoot the mark and have a stone that burns the undercrust in 2ish minutes, your only option is to get a thinner plate. If you undershoot it, then it's mod time (maybe a bump to 700)- and if you undershoot it too far, it's either serious modding time (maybe as high as 750) or, again, time for a difference thickness of plate. We could be talking multiple plate purchases and/or modding in order to match the bottom energy transfer with the top.