FFMPEG is not as good as it sounds...
The Amiga port supports only the AVI format and that is partially. Not all codecs are supported. That means that WMV, FLV, QT, etc to MPG is not possible.
Mencoder (part of MPlayer project) supports many video formats, it's more powerful and i think it's more fast too.
I'm not interested for a port of MPlayer because i've seen that it's too slow for a PPC equiped classic Amiga, but i'm definitely interested about a WarpOS port of Mencoder.