Which 15mm spacers are you using? Did you install extended studs or did the spacers come with the studs?www.willtheyfit.com
I am currently running 15mm spacers with the oem 18" wheels. I have my rear fenders rolled because my previous set of wheels required it, but now with the OEM wheels and their 50mm offset, they clear no problem. My car is lowered, and I do get a little rubbing in the wheel wells when making hard U-turns but the fender lips clear no problem. I cannot speak for the rears as they are already rolled, but they shouldn't have any trouble clearing the fender.
Ichiba version II 15mm wheel spacers. Bought them from Evasive Motorsports. Studs are pressed into them and they have low profile nuts to mount the spacer to the factory studs. Just be sure that your wheels have the open cavity to allow the protruding factory studs to sit between the wheel's stud holes. The OEM sGT 18" wheels do, not sure which wheels you have but just be sure before you buy.Which 15mm spacers are you using? Did you install extended studs or did the spacers come with the studs?