I have cooked many full size briskets on my 340, and have never had any issues. The only thing I do differently than any of the recipes I find, is drop my cooking temp about 20 degrees lower than they say. For some reason I always felt the 340 always runs a little hotter when the chamber is more...
On my 340 I would use the full setting when searing. However, I tried that on my 1250 and thought I was going to burn a hole in my drip pan. It was molten red. So definitely never doing that again.