Mpeg2 seen in dvd-video is not the best format for editing or in this case, applying filters to improve the quality. Normally if you know beforehand that the video needs to be adjusted, you would capture as an avi file, preferably with a codec that has little or no compression, like HuffyUV. Then use the appropriate filters in AviSynth, or VirtualDub or even AviDemux. After applying the filters, you convert to dvd-video. That would be the best path.
If all you have at this point is the dvd-video, you can try applying filters to your movie with something like Womble Mpeg Video Wizard DVD, since it can output as a finished dvd-video again and not make you go through an extra step to re-author. Using filters will mean re-encoding the entire movie, and each re-encode can cause a small amount of quality deterioration in itself. You'll just have to experiment and see if the results are worth the effort.
The Womble program does have a free, fully functional trial you can download. http://www.womble.com/products/index.html