I don't see much of a need to cook a 12-20 lb bird whole. In fact it makes little sense to me. Unless you do that whole 1950s magazine cover thing of surrounding the cooked bird with parsley and stuff and presenting the whole turkey to the guests before returning to the kitchen for 20 minutes to carve it. Which, of course, you don't do.
Even spatchcocking doesn't make much sense to me.
The goals of any method should be maximize the skin's exposure to the hot air, and, ensure the white meat doesn't overcook while waiting for the dark meat to finish.
Breaking down the turkey does exactly that. You cook the parts as long as you need to. If the breast is done, pull it out. If the wings are done, pull them out.
You're also getting rid of a large amount of mass and fat. The more mass that's in that oven, connected to that bird, the longer it has to cook. The longer it cooks, the more it dries out. This is one of the reasons you should never put stuffing in a bird. Why increase its mass? It'll just have to cook longer--not to mention there are some safety risks associated with putting that stuffing in the bird. And you don't need all of that fat from the back and other parts dripping into your pan. There's enough fat from the rest of the bird for your roux.
Another benefit is the carving. I really don't enjoy wrestling with a big, slippery, hot, round ball of bird while I'm trying to carve breasts off of it or remove legs. It's a hell of a lot easier to do that when the bird is cold and raw. I also take the opportunity to remove the bones from the thighs. Who wants to carve around those things? I then roll them up into little logs, which yield a lovely presentation once sliced.
And as a bonus, you can bang out some turkey stock with that back right out of the gate. I'll use it instead of chicken stock in the coming months because, quite frankly, it don't matter.
So break down your bird, you glorious freaks.