Geoff, Neapolitan isn't going to happen in this oven. Stacking might give you some slight NY charring in 2.5 minutes (@620), but if you want the best out of this setup, accept your limitations and embrace NY wholeheartedly. With KABF, I'd go with something like this:
Position plate 6" from broiler, along with some broiling (maybe 2 minutes out of 4)
Recovery time between pies (I'm thinking 7 min. minimum)
Brick/aged cheese that's been grated, not sliced (so it melts faster and you get more flavor from it)
Was the convection fan on for the whole time? If it was, then you'll definitely need to go with some broiling.
Flavor in the crust is a factor of fermentation (along with browning and char). How long are you fermenting the dough for?
1/4" steel plate can't do fast bakes at typical 550 deg. temps, at least not without abnormally high quantities of sugar. You're one of the lucky ones with an abnormally hot oven. You've got to be able to consistently hit those 600+ temps though, with the plate on a higher shelf.
If you stack, take a good look and see if there's any kind of gap between the stones. If there is, stacking won't give you much of a bump. As far as bottom heat goes, with some sugar and oil, as long as you can consistently hit 610ish, I think you should be fine without stacking. If you do stack, make sure you increase your pre-heat time accordingly. If your old stone is relatively thin, then you might be able to get away with 70 minutes, but I'd probably go with at least 90.