My 2.0T FSI S-LINE Special Edition rubbed all the time on 15mm spacers and that was with no suspension mods at all. Everything i went over speed bumps, any medium dips in the road, sometimes when cornering and definately when people were in the back.
i didn't however remove the magical screw that people now recommend. I now have 12mm on my S3 and all good....
12mm is safe bet.
yeah 15mm is to much, I had 16 & they rubbed like crazy so 15's wont be much different as I measured aswell for lesser ones which havnt got yet, its 12mm or below I believe from number people on here for rears & 10 fronts but fronts already good enough from stock, I believe.