If you live where it gets cold enough to snow, then winter tires are a must.
Snow is not the issue it's the ice patches.
All season tires are simply summer tires with a tiny bit more bite over ice, barely.
Winter tires are a whole other ball game over ice. You will be safe at faster speeds and over tighter turns. The rubber sticks.
As far as summer over all season, you have a car that is 3/4's 996.1 Carrera with the part that isn't Carrera being the best part of all (mid engine).
That's a world-class handling car. Putting it on all seasons for summer use is like wearing a custom made tuxedo with work boots.
One time I drove another car with all-seasons, after immediately getting out of my car (RA-1 r comps at the time), and I couldn't believe how numb the steering felt in comparison.
Enjoy your car the way it was designed to be used. This is a roadster not a bloated convertible.
Also, winter tires last forever, which in turn prolongs the longevity of your summer tires since you aren't running them year round.
This video sums it up well, particularly at 3:11
http://youtu.be/GlYEMH10Z4s