Well, for what it's worth, my 2.5# brisket cooked to 190 deg in 6 hours with the oven set at 230.
Yes, GLH, the AQ does max out at 300. I cranked mine up this evening to 300 and compared the temperature control with my electronic thermometer. It was spot on. I'm suprised how cool the outside of the oven stays with the oven at 300; very efficient. And yes, my chickens really did take that long. They started out refrigerator cold - they were 4.5#, never frozen, not stuffed - and they took 12 hours @225 for the breast meat to reach 180. I never did open the door, not once. I probably did overcook the birds a bit at 180.
I'll try your suggestion of cooking them at 300 the next time and call them done when the breast meat hits 170.
At this point I'm 100% convinced that the AQ electronics are performing perfectly for both the oven and the probe.