I’ve recently been doing some tests to answer this very question.
Conclusion: Avoid Compressor.
Use Quicktime Conversion. Animation codec. Highest quality, of course. In the “Size” field, enter the appropriate dimensions (i.e., 720 x 576 for PAL). Use BitVice to encode the MPEG, selecting 16×9 flagging. If your Dolby a.pack app has disappeared with the installation of Compressor, use Compressor to create just the .ac3 file.
Test this yourself with 2 seconds of footage converted by Compressor (various codecs) and Quicktime Conversion (various codecs). Re-import to FCP. View at 400% or 800%. Animation still looks the best.
Cheers.