RMR, I bought that book for my brother-in-law last Christmas.
I never cook a brisket fast. When I do brisket they cook for 16-18 hours (whole, untrimmed brisket that is). Sounds like you may have a trimmed flat, in which case it won't take as long. But six hours still seems like a short time to me. If a brisket cooks too quickly it will be tough. It is the long exposure to low heat, 225-250*, that breaks down the sinue and fibers in the tissue and makes it tender. I'd run it for about 8 to 10 hours and IF it finished early you can always wrap it in foil, then in a towel or two, and hold it in an insulated chest (like a cooler) for several hours.
A coke can chicken in the smoker by its self at 225* takes me 4 to 4-1/2 hours. Adding more mass (other meats) to the smoker at the same time increases the cooking time. Two chicks took 5 hours. Five at once took, I believe 7-1/5 hours.
Raising the temperature to 300* will shorten the cook time some, but remember that brisket will turn out tender only by Low-N-Slow. So don't try to rush it too much or you'll have a tough and chewy meal.