Even if your car comes with staggered tires front to rear as Original Equipment you may want to look at the same size all the way around for winter tires. BMW, for the new M3, recommends 235/40R18 on all four corners in th winter. The narrower tires are going to create more pounds per square inch on the contact patch and they are going to cut through the snow better. By running the same tires all the way around you are also able to rotate the tires and it will cut down on understeer as well.
Keep a lot of the handling in your performance car by choosing Performance Winter / Snow tire. The Dunlop SP Winter Sport 3D and the Bridgestone Blizzak LM60 would both be excellent options. So why park that car for the winter? Outfit it with winter / snow tires and let's go drive.