Regarding the STIFFER Swaybar, It wasn't put in to correct this condition, it was put in the elminate the tail wagging with the stock Swaybar that's installed in the cars that are built in the USA (20mm is the standard everywhere elso I'm told). This condition is pre-oversteer, it's dynamic rear suspension based in my view/experence. I installed it last spring so this is the first winter driving it with it installed. All I can tell you is that the condition is greatly reduced (but it always hard to compare winter driving conditions from one day to the other).
This is my third AWD car I have owned (88 Pontiac STE AWD - 98 Legacy (which doesn't have the issue) and the 05 OB Sedan). I live in Michigan and travel 40 to 50K a year for my work (Company supplies front wheel drive/non Subaru). I was an Instructer for 19 months for USMC at the Military Defensive Drive School in Quantico Virginia, so I'm pretty confident in my winter driving skills. I also owned a 76 Jeep CJ5 back when I was in the Marines in the 70's, which was the worst winter vehicle I ever owned (monster mud tires and a short wheel-base and snow don't make it).
The concerning aspect is this vehicle can't be driven at a speed that the other traffic is traveling at when these conditions are in-place. I've made the car better (the Dunlop M3's have helped also) but I would say it no where as good in these conditions as my 98 Legacy.