I've never had this size (although it's the size that comes on the Santa Monica's).
Standard GTi 205/55/16's should be 2.1 BAR front, 1.9 BAR rear;
Anniversary/R32 225/40/18's are 2.3 F, 2.2 R;
So I guess the 17's should sit inbetween those pressures??
Try 2.2 F & 2.1 R??