I think you opiontions are limited with the stock GXP size (that what I seam to remember from when I was looking for mine)
Im my opinion, even though it cost more, you should have a dedicated summer set and a winter set on steelies..
If you did, Id recommed the Hankook K104'*.for summer (the are friggin' awesome)
you should be able to drop to a 16"-17" steelie in the winter and get a good winter tire again, my recommonation would be Michelin X-ice. (100% best winter tire out there)
Just my opinion, I know its not exactly what your after