Alrightie! A Briskie Hissy Fit! Such fun!
I have cooked a 4-5 lb. flat at 275*, foiled the whole time, in the oven for 5 hours and it was done. It was tender and juicy. It was not smoked though. This was done as a test for the elusive "probe slides in like butta" texture.
So just to add to the fun here. Time and temperature will get you close to where the brisket should be, but to gauge the doneness, read tender and juicy here, forget reading the temperature and go by probe feel. It's done when it's done. My personal best packer brisket was cooked on the grate, no foil and no thermometer. No idea what the internal temp was either. Probe a few times and when all resistance was gone, took it off to rest. Separated the point and flat with a gloved hand only, no knife. The point was drop dead delicious and the flat was tender and juicy. Hopefully I can replicate this cook. Anyway, getting wordy here, so I'm done. Good luck with your cook and let us know how it goes.