Nah, you can drive in the snow in Conti's. Of course, you get about as much traction as you do spinning innertubes across the top of a pool.16" is required. The stock Conti's are a summer-only tire so you need to either go down to a 16 or put snows on the stockers.
You are correct - I used the Tire Rack calculator (and it calls out the SVT, not just a ZX3 or 5), which gave the 205/50. Running the numbers manually just now though, I get a diameter of 24.618" for the stock rubber, and 24.071 for the 50s and 24.878 for the 55s. So, the 55s look closer to me - Tire Rack is wrong.I have 205 55 16" as the correct Winter size from the SVT manual supplement. I imagine 205 50 16" is the standard ZX3 size, but other than the standard ZX3 size being a bit smaller (I think), is there any reason the 205 55 16" was recommended by SVT for our cars or won't work?