Steve, first of all, within the last decade, there has been a disappointing trend towards energy efficiency, which, in turn, has created weaker ovens. Ovens are getting prettier and more feature rich, but they're not getting stronger- or even maintaining the strength of years past. It's not critical, but, if you get a chance, try and find the wattage for the old maytag and compare it to the new oven. I'll bet any amount of money that the maytag was considerably higher.
Richard (Sqid) brought up a good point recently about convection circulating heat that's already in a system, but it's not adding heat. This is why your initial one or two pizza runs turned out so well- the convection is taking the heat in the system and transferring it to the pizza in a record time, but... convection isn't bringing more energy into the picture. It's the bottom element that's responsible for that, and if that element is weak/low-ish wattage, then it's going to take a while to replenish the stone and bake a large number of pizzas in succession.
Anyway, that's the theoretical stuff. Here's some concrete steps you can take that will help the situation:
1. Use the steel, on it's own. It will have considerably more thermal mass than your present stone and be able to store more heat. It will also replenish faster because of it's higher conductivity. 1/2" steel preheat to 550, on convection, for 80 minutes, will give you plenty of heat for 5-6 pizzas. Btw, your oven has at least 2" clearance on each side of the plate for air flow, correct?
2. Extend your pre-heats. I noticed on your earlier thread, you were going with a 30 minute pre-heat. Even if the outside of the stone is 545, there's no way the inside is that temp. Complete saturation with heat is critical both for fast bakes and fast recovery. It all boils down to how many watts your oven is pumping out, but until you have the data, I would probably stick to 80 minute pre-heats.
3. Make sure that during the bake and during the time between pies the bake element is on and the convection fan is running. My stone stores enough heat to do 3 pies, so I can turn my oven off between pies, but your bake element should always be on.
If you want to try an 80 minute pre-heat on your current stone, it might prove interesting, but I would just step up to steel.
I've always dissuaded people with broilers from using two stones, because the top stone generally won't shower the pizza with as much heat as the broiler will and the bottom stone will shield the top stone from the bake element below. This being said, with convection theoretically moving air through all parts of the oven, it's quite possible that convection might allow you to bake on two stones- that's how the rotoflexes work. Still, I think you should be perfectly fine with steel and an 80 minute convection pre-heat. There's a really good chance you'll need less than 80, but I would start there and work your way down until you start seeing longer bakes on later pies.
If you ever hosted a party and needed dramatically more output, I would give two stones a shots. Position the stones towards the center, with plenty of space in between for good air flow. Put the ceramic stone on the bottom and the steel on top, since there's a chance the ceramic stone will end up a bit hotter. Go with a 2 hour pre-heat to make sure both stones are fully saturated. Use convection during the bake and between bakes.