I have been testing and trying many different ways to answer this problem. If you have HD footage in FCP, and want it look good on SD DVD using Studio Pro, you may find that this is helpful.
Go into you FCP for your sequence settings. Change the compressor to ‘None’. Export Quicktime. This will give you an extraordinarily large file, (like 100+ gigs, depending on your project). Take that file directly into Studio Pro, and NOT compressor. Allow Studio pro to encode for you.
Couple things to think about. This works for smaller movies in length, as the gigs do not exceed reasonable storage and usage. You must adjust your encoding settings in DVD SP as well. Start with a high bit rate, and when it says that it is too high, slowly turn it down, until you find one that works. You may have to change this in the preferences of SP, and re import every time it says its too high.
All sounds very confusing, but it you will be pleased with the results.