So, I reheated the brisket in the sealed bag, for about 30 minutes over a simmer. It did have a finer feel and texture to it, and felt more tender. I think you might have been right Kevin, I was too worried about the temps of 210 measured in the flat. Next time I will just ignore the temps, and go with time and feel.
I am up in Chico visiting my dad, and will get to use his Stumps Clone BBQ, tomorrow night into Wednesday morning doing one of the primes I bought. I have already done a slight trim, and salted and rubbed it, now it's chilling out in the fridge. So, it will have 24 hours of time with the dry rub (no mustard) before it gets cooked.
I had originally planned on serving it for dinner tomorrow, but realized there was no way to have a 16.5 pound prime brisket done properly over low temps (180-190) and have it ready to serve by 7 pm, without going out and firing up the Stumps now.