What Z4 do you have? Non M? If so, 15mm may well catch on harsh bumps.
I have 12mm on the front of mine with eibachs springs and tbh it does get pretty close to catching the plastic liners even when sitting on level ground.
I have 108s and 10mm H&R spacers which still catches ever so slightly on the liner by the reflector. I've never noticed it whilst driving but there are small marks. 1.25 degrees of camber.