I'm based outside of Chicago, and am looking at getting a 991 for full-year driving... I've had a C2S previously, but drove an SUV in the winter at the time, and I've had a C4S I drove full-year... Assuming in either case I have a set of winter tires, can a C2S be a good solution during Chicago winters, or do I really need the C4S?

 

C2S with winter tires is fine in Chicagoland.
I wouldnt hesitate.
I've done it for more than 10 years

 

Winter tires are cheat mode, more than AWD.

AWD would help more with hills, a really steep driveway, those types of problems. It's easier to get unstuck if your car gets impounded in a drift, but unless you plan to park outside during big snowstorms that may not be much of an issue.

I had a Subaru (04 STI) with both winter (WS-50) and all seasons (Bridgestone 960AS) at different points in time. I had winter tires (LM-22) on a 135i the two seasons I actually drove that in winter. Both ~3200lbs 380-450hp+ (tuned), 6MT. The winter tires are more important 95% of the time than driving all the wheels.

Blizzak LM-series are good if you're mostly driving on plowed roads. A good portion of good winter performance is simply having a rubber compound that doesn't turn into glass when the temperature drops and the LM does a good job there.

Blizzak WS-series if you are more out in the sticks as they have more aggressive tread cuts.

AWD can be a lot of fun, though!