I cooked a packer brisket 2 weeks ago on my WSM. I put it on at 11:00 am, then went out to run errands. I used the same amount of charcoal and set the vents as I usually do to cook at 250. No water in the pan. I got back at 5:00 pm and the temp was at 317!! I battled to get it down but it never got under 305. So I let it ride between 310-325. A few hours later I had a nice wobbly brisket and by far the best brisket I've ever cooked. I don't know how long it took to get up to 300+, but It was there for a while. I can't wait to cook another brisket hot and fast.
18.5" WSM, 22.5" Weber Kettle, Weber Smokey Joe