Why 7mm & 14mm wheel spacers?
#16
i'm running 285/30/18 on the rear of my narrow body 01 C4 and there are no issues.
#17
Correct! You'll be fine with 265/35/18 on a stock 18 inch 10 inch wide rear wheel with a 15mm spacer.
#19
John, I hate to say it, but it just seems like the 15mm on the rear make the wheels stick out too much on your C4S. From my recollection, 5mm all the way around on the Mk1 gets you into GT3 spec at the time.