I've not noticed any dropoff. In fact, because the brisket and butts are sitting in their own juices (partially), I believe they are actually soaking up more liquid after the cook as they rest down. It does make it very nice to not have to wake up in the wee hours to a potential bad surprise.
I don't get any extra sleep cooking early & holding, I actually loose sleep. The meats don't get done until around midnight, and I have to get up around 4:30 for the ribs. Doing it the more traditional way, I could put the meats in early, go to bed, and get up at 4:30. Of course I did have to get up at some point to foil.
For pre-heat, I've been just using a pan of near-boiling water sitting on the bottom of the Cambro. But I think that putting two brisket and two butts in there that pre-heating really isn't needed. The water condensed all over the interior also makes a big mess to clean up as everything becomes wet. I do however remove the pan right before putting the meat in.
That all having been said, I think I'm going to go back to cooking overnight and finishing up by about 6:00. Even though I don't think there's much if any fall-off in quality, I think I'm convincing myself that more freshly cooked just has to be better.
Russ