The gas in the flue flows over the dome right as it exits the oven. I don't know what the temperature is, but I'm guessing it is quite a bit hotter than the bricks right below it. If this is the case, heat will flow from the flue gas into the dome - not the other direction. Moving the flue so that it sends the exhaust straight up may be counterproductive as the heat that was going into the bricks would be lost. Is this not correct?
Craig, the flue gas travels out the door and then up the flue, so, by the time it reaches the area above the oven, it's going to be a bit cooler than the dome bricks, and thus draw heat. Because the gas is moving, the convection will increase that slight cooling effect. This being said, I don't really foresee the flue channel being that much cooler, especially if it's insulated above it.
The looping Neapolitan vent has to be giving the Italians something or they wouldn't be doing it- and I'm sure it's more than just cosmetics for them. I think we've confirmed that it gives them better fuel efficiency, but, at the same time, if it did cost them in some area, such as peak temp, I don't think they'd pay that price for fuel efficiency- and I've never seen a Neapolitan oven with peak temp issues. I also don't see residual heat issues in Neapolitan ovens either. I don't know, perhaps there's a slight hit in residual heat with this kind of vent, but I don't think it's enough to do away with it.
Lastly, there could easily be an unknown facet of the thermodynamics that we're overlooking here. I don't worship the ground that Stefano, Mario and Gianni walk on, but, at the same time, unless we have cold hard data revealing the inferiority of the Neo vent, I think we should give them the benefit of the doubt.