I exported a ProRes master file from FCP7 to import into FCPX for color grading. Then exported a ProRes master from FCPX and fed that to Compressor 4 (current version) for multi-pass, max bitrate encoding to a BluRay-compliant format. (Source material was ripped from BluRay, after all.)
Can you give any examples of compression artifacting you noticed? I can compare and see if they’re in the source, or the ProRes master file.