be sure to check for tender in multiple spots. The point is extremely well marbled, so the fat in there will seem tender, and the very tip of the flat seems to get to tender before the middle. Probe around every 20 min or so, and keep cooking it, and you might be surprised at what you find.
When I'm having troubles with getting a tough cut to tender, I try to go as hot as I can, and do more probing so I can get an idea of what's happening. L/S can make changes in tenderness almost imperceptible, and you'll panic and pull it too soon because nothing's happening. Every cut I learned to cook, I started out HH and then backed down on the temps on later cooks once I knew what I was looking for. To learn a cut, you need to see it both underdone and over done, so go ahead and practice.
Personally, I'm still not sure if I've ever overcooked a brisket. The last one I did was a 12 lb packer that cooked for 8 hours at 275* and I took it further than I ever had and it was the best brisket to date.