Gabz: the conversion CMYK-RGB works automatically in PhS.
You need the CMYK source profile (or a similar, e.g. SWOP,
Euro Uncoated...).
All conversions use the CieLab color space as a physical description
for the colors.
RGB to CMYK via CieLab is lossy (all colors are modified
in order to match the printed colors as good as possible to RGB,
the so called gamut compression).
CMYK to RGB via CieLab is not recommended. The gamut is ONCE MORE
compressed. -----Gernot