Warning DO NOT use compressor and expect it to encode the settings you make in the options panel of the x.264 FFmpeg codec.
I just finished a round of testing and can say with certainty that no matter what you set it will not be passed on to compressor. It will be completely ignored in the final encode. The codec is used but the settings you set are ignored. This has been reported to Apple Professional support. To test it yourself you will need something like the MediaInfo app to read the file (.99 cents on the app store). In order to test this yourself you will need the ffmpeg x.264 QT codec installed. You can get the codec here: http://www.macupdate.com/app/mac/24173/x264encoder. After installing the codec follow the steps below.
1. Open Apple Compressor select a new quicktime component.
2. Under the video tab in the inspector choose change quicktime settings.
3. Choose the x.264 encoder from the compression type drop down.
4. Select the options button in the window.
5. You are now in the libavcodec settings window. You can do anything you want in here and it will save your settings but when you output your file it will not have used anything that you have set in here.
Try it and then open the file in Mediainfo. You will find that the settings were based on the size of the file and the data rate that you set. Nothing inside of the options window influences the encode.
Try the same thing with QuickTime Pro 7 or use MPEG Streamclip. If you open the resultant file with Mediainfo you will find that everything you set in the options area has an impact on the encode.
If you need the high quality of the x.264 encoder and need to do batch encoding then I would recommend using MPEG Streamclip. Compressor gives you the impression that it is encoding the way that you set it but indeed it is not.