Oz, how new is this oven? It might just be a matter of all the moisture not being driven off yet. How many times have you fired it?
I would, as others have stated, take a long hard look at how the oven was insulated.
I took a look at the gr 140 web page, and, unless the photo is wrong, these are not the massively high domes of a typical Pompeii. They're not low dome either, and I'd never recommend them to someone shopping for a new oven, but, for someone who has already pulled the trigger, there is anecdotal evidence that Neapolitan pizza can be coaxed out of these in a commercial setting, so I wouldn't immediately throw in the towel and start doing NH pies (not that there's anything wrong with NH).
Looking at the size of the door, I think, like the FGM, this could benefit from an inner door insert. While an iron insert would obviously be ideal, I've been thinking about DIY ways to implement a lower door. One could cast a piece out of refractory, but it seems like more work than necessary. The insert, imo, really doesn't need thermal mass. Theoretically, if you could find a way to attach copper flashing over the top 1/3 of the door (with an air tight seal), it would help keep a little more heat in the oven.
I've also been thinking about a raised floor for these kind of high-ish dome scenarios. I know that Bill played around with raising his floor and wasn't impressed by the results, but I think he just added bricks. I'd like to see someone add bricks along with a layer of insulation.
Jeff proposed a theory a few months back that still needs to be tested- that conduction from the hot coals in the side fire is responsible for a great deal of hearth heat. If this is true, then an insulated brick shelf for the fire would allow for higher dome temps without driving up the floor temp quite so much.
I guess what I'm getting at here is that just because the Valoriani's aren't ideal out of the crate, perhaps there are mods that can help them perform a bit better. If someone, like Oz, has already made the commitment and can't return the oven, it can't hurt to try.