I know I'll probably hear from some other "winter" drivers here. But with 43 years of North East Ohio (lake effect, snow belt) driving experience under my wheels, I've found the vehicles I've owned that didn't have a "posi" rear end actually went thru the snow better. Especially in a car with a very similar weight, wheel base, & weight distribution as the Crossfire has.
A sure-grip in the Crossfire on a snowy road=doughnuts.
I highly recommend a "tall, skinny" Blizzack snow tire, (preferably) on all 4 wheels, and around 100lbs. of weight in the trunk.
If I had to drive my Crossfire in the winter, I'd pick up a set of 17" wheels from a SLK, thru LKQ Salvage, and turn the XF into a little CJ5.