Re: Distribution downgrade and now poor performance with h264/mplayer/vdpau on G210
demuxer=lavf does cause problems with some file types, and as I said lavf is already default for most appropriate types in mplayer2.
What "preferences" would those audio codec settings achieve? Many of those are the default decoder for the corresponding format already; adding such stuff explicitly to the config file is only asking for trouble in the future. Was there ever an actual problem solved by each of those?
The forcedsubs setting means that in many cases English subtitles would not be visible for videos with foreign-language audio.
Enabling de-interlacing does have an effect regardless of whether the content was originally interlaced or progressive, and enabling it for non-interlaced videos is definitely a bad idea.
The surround filter will have no effect if the number of input channels is not 2. In the stereo case it will modify audio whether it was actually matrix-encoded or not.
Setting autosync is generally a bad idea unless you have a specific audio output problem requiring it.