You also need to consider what's on which rack.
On my last butt/brisket cook, I put two big butts on the bottom, maybe about nine pounds each, and a thirteen or fourteen pound brisket on top. I cooked all night close to 250* at the vent, water in the pan, and surprisingly, all were done at about the same time and sooner than expected, just 12 hours later...no foil or turning/rotating. Obviously, the extra heat between the pan and the cooker's wall is what made the butts get done so fast.
Thankfully, butts are quite forgiving of the uneven cooking down there on the bottom rack, at least when I keep the cooking temp around 250 or lower. By contrast, as to temp/time, when I "spoke" chicken leg quarters on both racks but only have the bottom rack chicken over the water pan (not over the "hot zone"), the top rack chicken on average gets done first. As to the temp difference, on my last cook, a little oven gauge read 275* in the "hot zone" right next to the door when my therm in the vent was reading about 235*. This was in the first part of the cook, though, before the chicken had really started cooking good and the top rack was pretty full.
