I ran into the same problem while running 15mm spacers in the rear. I also lowered the car with H&R springs and the car itself was a ZCP model.
It rubbed on bumps but that was enough to concern me because my rear fender sticks "slightly" out a little more now. I switched the rear to 12mm spacers and after that there was no more rubbing! I was aware that I could have tried to roll my fenders but I just didn't want to go that route to reduce my cars value if i trade in.
As far as looks goes... the 15mm definitely looks better... it was pretty surprising for me to see that the 3mm made a difference to me. Nevertheless... it feels a lot better to be able to drive without having to worry about rubbing!!