I dunno, your brake bias would most likely be out of whack with that sort of setup. Best to just go the whole hog and do the Gravel Group N setup of the Subaru 4/2 setup and use the standard 15" gravel wheels.
Otherwise, STi owners are just using 17" snow tires or an open tread M/S like the KDWS. Advantage of the KDWS is the much stiffer sidewall compared to any snowtire, which allows you to run a bit less tire pressure compared to a snowtire to prevent tire roll off.