Good question. I'm curious too! In my experience, the briskets on the top grate cook faster than those on the bottom, even though the lower ones are closer to the coals. I suspect the water pan is acting both as a convective air deflector and a radiant heat shield.
What I'm curious about is how that third grate will affect airflow. Will the middle grate's briskets get the most heat? We'll see.
I'll try to document everything with photos, metrics, and final results.
My goal is to develop a solid bark and smoke flavor (maybe around 170 F) before finished them in my oven, followed by a long hold at 150 F, for 10 or so hours until the event.
I'm not too concerned about them being in the danger zone. The brisket surfaces will surpass 140F within the first hour, and the smoke should inhibit bacterial growth. I'll also rotate the briskets during the cook and closely monitor the coals to avoid any temperature dips. I won't be adding water to the pan, as six briskets should release plenty of moisture on their own.