UPN (back when he was in NYC) was one of the first great "Neapolitan" pizzas I tasted. It inspired me to spend a considerable amount of time working on a faithful reverse engineering of his dough
http://www.pizzamaking.com/forum/index.php?topic=10237.0 My vision of Neapolitan pizza has changed a great deal since then, notwithstanding it will always be pizza I remember fondly which is probably why I have no desire to go to UPN/SF.
Like Bill, I tend to doubt the sawdust does much for the heat and may be largely for show though I think there may be another reason I'll discuss below. In addition to the reasons Bill mentions for the added heat to be relatively trivial, the burning sawdust puts off considerable amounts of black carbon soot and smoke that hang in the air above the pizza - you can literally see a layer above the pizza - and this absorbs a lot of IR from the dome preventing it from reaching the pizza.
I don't think he always uses sawdust. He didn't do it when I was at his place in NYC. Seems like I've seen more than one video where he doesn't add sawdust. I'm wondering if he does it when the oven is underheated trying to add some additional heat? The pizza in your picture above looks like the oven might not have been very hot. You can see there was very little top heat on that pie. Almost all the black on the top of the cornicione is burned sugars from the sauce and not charring of the dough.
On multiple occasions, I have tested different woods (oak, pecan, and mesquite) using dough from the same batch, in two WFOs running side-by-side - burning only one wood for the entire firing. I'd argue that this is "the only real true test." Granted the two WFOs are quite different, so it's not exactly scientific. Notwithstanding, I'm very skeptical that the choice of hardwood makes a perceptible difference. There are plenty of factors that are literally many orders of magnitude more important - oven temperature and heat saturation, and dough ripeness and temperature being just a few.