I don't think the engines have anything to do with delays on the R, they can assemble more than 100 5.2 engines per week and those are starting to ship regularly on the base models. Most likely the planned schedule to hold off if Ford can't send cars out due to the PSS2 tires in cold climates, why build them and have them sitting around for 2 or 3 months. They can't store them outside at Flat Rock either. Good Luck with your order and hang in there, it's worth the wait.I imagine that the sourcing of things such as the hand built engines and carbon wheels cause delay.
My order status has progressed from "entered" on 10/15/2015 to "locked" 12/29/2015 with a partial VIN, according to my dealer.