Finally I found the time to write down, how I use Ghostscript:

gs \
    -sDEVICE=pdfwrite \
    -o /output/gs_file.pdf \
    -dPDFSETTINGS=/prepress \
    -sProcessColorModel=DeviceCMYK \
    -sDefaultCMYKProfile=/path/to/ISOcoated_v2_eci.icc \
    -sColorConversionStrategy=CMYK \
    source/file.pdf \
    -f

I don’t now which of ProcessColorModel or ColorConversionStrategy is the important one. I kept both and did not bother to try to omit one of them. -dPDFSETTINGS=/prepress makes sure that embedded bitmaps are in 300dpi and I think -f prevents Ghostscript staying in interactive mode after all pages have been finished.