FFmpeg is only used if a video requires transcoding, which most modern video sources won't need. I've found that the compression we use when we transcode videos is fairly decent.
There is, however, a gap in terms of videos that do not need transcoding. If we make a change in this area, that will be in XFMG 2.0.
In the meantime, you can either edit the code or it would require an add-on.