If you are depending on your Boxster to get you around in snow and ice, I would highly recommend getting actual snow tires. It makes a huge difference. I am using Continetial ContiWinterContacts, which are Mud & Snow rated. And then you can have dedicated summer tires.
Summer tires below 40° (especially ultra, max performance) can be very dangerous, because the rubber hardens, so it is like ice skating. All seasons are not really good at anything. They are not good in snow, they are not very fun in the summer. However, if you are not a spirited driver and have access to another car for blizzard situations, all seasons could be an acceptable option.
-Rick