Don’t ever let FCP scale a HD image down in a SD timeline.
Best workflow here i think:
Export a quicktime movie with current settings in HD.
Open in Compressor.
Create a ProRes 422 or 10 Bit uncompressed SD preset.
Make sure you have frame controls turned ON, resize filter to BEST.
Submit.
Import the file in FCP.
Now you have a great looking SD timeline which you can dump to DV through firewire.
Or to Digibeta through a SDI board like AJA, Matrox or Decklink.