I have also noticed a drop in performance toward the end of the tank. But this is with a full tank. Do you have much of a difference in flame between 1/2 open on the knob and all the way open?
Tommy, rather than take Dylan's thread too much off topic, I thought I'd present a few ideas here.
First, in Dylan's thread, Dave (Tampa) posted a link to his incredibly useful de-covering post. Just in case you missed that, here it is again:http://www.pizzamaking.com/forum/index.php?topic=26507.msg279722#msg279722
There was someone else on the forum that drilled out the rivets on the cover, but the name of the person escapes me. I went back 10 pages on this thread and couldn't find it. Anybody remember?
After you remove the cover, I'd take a close look at the air feed and make sure there aren't any obstructions in the air intake, such as cobwebs. I'd also compare your opening size to Dave's.
This isn't something that I'm in the position to help with, but I'd like to see people start testing fuel usage by weighing their tanks, running the oven at full bore for a set time (20 minutes?) and then weighing the tank again to determine BTUs. Not only would this give you a clear perspective of where your burner falls in the spectrum of BTU output, I think it would be hugely helpful in comparing the different permutations of oven. I would be especially curious to see these numbers for a 5 psi model vs. a 10 psi one.
You just need one person with a 50+ lb. digital postal scale that can do this. Dave's sort of moving into a Blackstone tech guru/company liaison role, maybe this might be something he'd like to volunteer for
It need not even be separate from a typical bake. Just weigh the tank, full bore 20 minutes, weigh the tank and then bake the pizza. I'm going with 20 minutes since that seems to be a typical pre-heat, but, the longer the better, as that will decrease the margin of error. You shouldn't need to remove the hose from the tank, either, as long as you're careful to weigh it in the same position.