In preparation of my new seminar PDF Color Management I have tested the image compression before and after color conversion in the most popular PDF tools.
In highend PDFs destinated for quality printing the images are usually compressed using JPEG maximum quality. Before color conversion the images must be decompressed and afterwards compressed again. Acrobat always uses JPEG medium quality for this recompression. The consequence is that there is an (unexpected) loss of quality of the color converted images (see table).