The Marsal MB is 120K per deck, so you might be at a bit of disadvantage, depending on the size of the decks. What are your internal deck dimensions?
I don't think this mod is terribly costly, so it definitely shouldn't hurt.
This is a bit of a hassle, but, if I had this oven, I'd (carefully) take out one of the stones, weigh it to get a density and compare that mass to 2" firebrick. As Pizzaneer pointed out, you'll take a conductivity hit with the firebrick, but... if 2" gives you considerably more thermal mass, it might be worth it.
I would go ahead with the thermostat mod, install the firebrick ceiling, crank the oven to the max and see how hot you can get the floor and how the top and bottom heat are matching up. Once you have that information, then I think we'll be in a better position to help you choose a better hearth, if necessary.