I think it holds true that the better the piece of meat you start with, the more likely you will end up with a great result. But, that is not to say grading the the ultimate answer. A Choice packer with a 1" thick flat is not necessarily going to do better than a Select packer with a 1.75" thick flat. One thing to remember is that the grading does not hinge on brisket quality, so it pays to select the best meat you can get, regardless of grade.
I happen to cook several briskets a year on my kettle and several on my UDS, and I have a reasonable chance of producing an exceptional brisket on either cooker. I don't happen to think it all comes down to temperature, it comes down to temperature, moisture in the cooker, cut of meat, airflow as well.
I will tell you, I think more briskets are ruined due to rushing, not allowing enough time, not resting long enough and insisting on using internal temperature than for any reason that could be traced to cut or grade of meat.
me: I don't drink anymore
Yelonutz: me either, but, then again, I don't drink any less