The GT3 wheel option M416) on the C4S and Turbo are as follows:

8" x 18" - ET50 - 996.362.136.02
11" x 18" - ET45 - 996.362.142.02

Also, no spacers are required or used. In addition, the GT3 wheels are lighter than the solid spoke C4S wheels. I would definitely leave them on, if I were you.