According to the workshop manual on the MK7 I have, for the FNc-M42 (17”) discs fitted to the "R" and "PP" (310mm diameter), the new thickness of the disc is 22mm and the replacement thickness is 20mm
Also, the brake pad wear limit (not including the backplate) is listed as only 2mm (11mm from new), so maybe your pads need replacing, but unless the rear discs are badly scored or corroded, it doesn't (based on those figures) look like they need replacing ?