My original comment was not intended to be a critique of your oven - rather a correction of a misstatement by synaesthesia. For fast warm ups and 300C/5minute pies, I have no doubt it is a great oven.
Thermal mass aids heat retention and re- radiation, you can hit high temps by the strength of a sustained fire alone. It will be less able to retain heat as long as a masonry oven but you do not need that condition unless you are catering regularly on a commercial basis.
While it's obviously correct that mass retains and re-radiates heat, it's an incorrect assertion that the primary function of the mass is to retain heat because you need it for commercial volume. This isn't a pizza stone we're talking about.
You got it partially correct when you wrote "the only purpose of the upper thermal mass is to top brown the pizza"
But, it's more than that however. Its something you may not really appreciate until you try to push your oven up into the mid 400's+ (C). As you noted, the hotter you go, the larger the fire you will need because you're bleeding so much heat without suitable low thermal conductivity mass or insulation. As the fire gets larger, it becomes more and more difficult to balance the heat on the parts of the pie not directly in the path of the IR coming from the fire. Convective heat is not the same as radiant. Radiant heat increases with surface temp^4. As the mass of the dome and walls increases in temp, radiant heat increases exponentially. Convective heat transfer increases linearly with temperature. The hotter you get, the more pronounced an imbalance becomes.