Have new 18" OEM wheels in.
I wanted to stay at 17s for weight reasons but also cheaper tires. Ended up getting these 18" Cayman S II wheels for a decent price. I am planning to get either General Gmax AS or Conti DWS06 tires to run the car all year long-ish. Both tires are the lightest by a fair margin, if the data is true on their websites.
18x8 are 22.0lbs (9.98kg)
18x9 are 23.4lbs (10.6kg)
I sent this information over to the email on that 944 page.
Terrible pic of the front wheel:
Front Wheel, bathroom scale:
Rear Wheel, bathroom scale: