I see three excellent questions here:
1- How to cook a turkey without drying it out.
How do you keep turkey from getting too dry?
The quick answer to this question is always brining. I love brined turkey but often the drippings are too salty to make a gravy which is a tragedy. For that reason, and because of the faster cooking time, I wouldn't brine a butterflied turkey- or at least less than usual.
Basting is unnecessary and will not help the meat stay moist. It will add flavor but if the meat is cooking in its own juices this won't be necessary.
2- How to cook a turkey without monopolizing your oven.
I use Alton Brown's turkey roasting technique. He roasts at 500F for some time to fry the skin and then drops the heat for the actual roasting. If I am cramped for oven space I will do the 500F in the oven and then move the turkey to an electric roaster for the remainder of the cooking time. It doesn't brown as completely but the meat cooks beautifully.
3- The question you actually asked: have I ever butterflied a large turkey.
Of course the larger mass will take longer to roast but it will still be faster than traditional roasting so I wouldn't worry about it become too dry. The recipes that I found after reading your question sound delicious:
I think you should go for it.
Thank you for asking this question. I am definitely going to try this with one of my turkeys this year.
** EDIT **
I did it and it was delicious. Besides the faster cooking time and resultant juiciness- I also appreciated the greatly expanded surface area that allowed me to get more flavoring against the meat.