(3) 50 pound bags of kitty litter in the trunk, a full tank of fuel and rear tire psi lowered by about 5-7 pounds. I've never had an issue and have used the same "technique" during many years of driving high performance and stock Mustangs during NJ winters. I even used the same method in my prior 95 M3 and do the same with my 94 Cobra as well, never an issue. I'll be doing the same w/ the 318ic this winter. I've never purchased or had a separate set of winter tires, ever.
Just take it easy during such a storm, it's not how fast you get from point A to B, it's how common sense gets you home safely if/when there is snow/ice on the ground...
Besides "technique", I've heard and read many good reviews about the Blizzaks - good tire for sure, especially if you are in an area that experiences MUCH snowfall during Dec-March...