As you can see, when the door is too high, less of the superheated air is trapped in the dome. With less trapped superheated air, you get less heat transfer to the dome. The lower the door, the longer you can keep the heat in the oven, the hotter the dome gets. We're not talking about a huge amount of time- the hot air might stay in the oven just a fraction of a second longer, but it's enough time for the exhaust gases on oven #1 (higher door) to be hotter than the the exhaust gases on oven #2. Hotter exhaust gases translates into more heat loss on the high door. For the properly sized door you're talking about a faster pre-heat and a hotter dome overall.
Craig, since the door serves both the duties of exhaust and fresh air intake, you do have to be careful about how small you make the opening, but that's where the 63% ratio comes into play. The Italians have been testing this for centuries and that's what they arrived at. Like Jeff, I'm not in love with that exact figure, but I am head over heels for that realm- maybe 55%-70%, depending on the size of the oven, shape of the door and other factors. Above 70%, though, and you're flushing wood down the drain.
Now, a high door, like a high ceiling, can definitely be worked around. If you throw enough wood at it, it will get plenty hot. But if a simple insert can make this oven more efficient (and I'll bet any amount of money it will), then I strongly feel that it's worth adding.