The timing is no linear in a braise. In the Nightrain experiment, you are braising and the amount of time it takes to get done will not track in a strict mathematical ratio due to the vagaries of how mass takes and transfers heat as well as the moisture content and shape of said mass. Oddly, the amount of time cooking the 8lb brisket roast would have been very close to the time for the 5lb roast. As you heat the moist environment in the foil, you are actually raising the temperature of the liquids inside of the roast at the same time. Given that the larger flatter shape is going to heat from all sides at the same time, and given the flat shape, the interior moisture is heating at about the same rate despite the weight difference. This means that the actual rendering of the connective tissue will begin to occur at about the same time, and the stall as connective tissue renders will be about the same. The primary difference being the total amount of connective tissue rendering.
In a smoker, this actually also applies. Hence, the mystery of how a 10lb packer and a 14lb packer can both take 12 hours to cook. Part of the problem with using time, versus feel, is that the individual composition of meat varies widely (hence the 22 hour pork butt). Heat penetration is influenced by mass and shape of meat. Oddly, mass of a given roast is determined by both density of tissue and amount of internal moisture.
The major difference in timing is actually in the composition of the meat, not the weight alone. You're more likely to have succeeded with a 5.5 hour cook time over an 7 hour cook time. I would give it another shot. This time with the shorter cook time. And a smaller cut of brisket to save a little money.
