Guys, don't forget many of us waiting for cars now, ordered when production of the F30 wasn't at full speed. Cars sales are down something like 10% overall, so is possible the factory can cope with a shorter lead time.
However, even if they can't deliver by end December:
a.) The 3.9% apr deal may well be extended (already has been from 3/4 quarter)
b.) Dealer might be able to swing the deal to compensate for a higher apr if this happens.
Either way you'll still highly likely get your cars for the amount agreed upon.