My first direction would be to use whatever the OEM says to use a snow tire. They've confirmed it works well with the systems in their car on actual snow and ice surfaces. Doubly so since the car is a summer tire only car off of the showroom floor.
Second is, if you go with a different size, make sure the overall height is as close to the same as reasonable. Like Tom said, that aftermarket tire you suggested is much taller than the OEM tire. That big difference in tire height could cause performance loss issues with the electro-mechanical systems in the car not working quite right (esp ABS/TCS). If you are far enough off, you could throw an abnormality code, but this size shouldn't be far enough off to cause an issue.
http://tire-size-conversion.com/tire-size-calculator/
At the very least, your speedometer is going to be incorrect. When you put a bigger tire on it's the 'bad' way (speed reads lower than actual). Easy way to get a speeding ticket. It's always a good idea to confirm speed accuracy by GPS if you put different sized tires on the car.
Could you put a low boost tune on it? I wonder if Brian could do a very quick and dirty 'tune' and just change some max/requested boost 1 parameter change. Could make a very nice 'winter' setting on the car. Especially if the drive is not used to high TQ FWD (boosted) cars.
It looks like Ford says you should use a 195/50r16 http://www.fiestast.org/forum/fiesta-st-discussions/315-warning-fiesta-st-owners-about-winter-tires.html
Looks like for about $850 from Tire rack you can get tires, rims, mound & balanced with TPMS sensors http://www.tirerack.com/snow/WinterPackageMain.jsp?autoMake=Ford&autoModel=Fiesta+ST&autoYear=2014&autoModClar=