TLDR:
The first improvement (Media Foundation by FFMPEG) could be significant. Currently, VALVe generates large shaders to re-render those Media Foundation videos into other free codecs. These shaders can be several gigabytes in size for some games with lengthy videos. With FFMPEG, those videos could be played without being re-encoded as shaders.
It isn't significant. Wine already supports the vast majority of MediaFoundation codecs with GStreamer. This is just an alternative backend that uses FFmpeg instead of GStreamer. GStreamer already has an FFmpeg plugin, so this doesn't add any new codecs to the table. It seems there's just a long term plan to move away from GStreamer for whatever reason.
Wine's MF support used to be much worse, which is why Valve had to do their workaround shader hack. Not sure what exactly the current status on that is, but I do know things like mf-install or Proton-GE are rarely if ever necessary anymore, even with non-Steam games (which I have plenty of).
Digging into the GitLab & related discussions, the main takeaway I got is that FFmpeg's API supposedly meshes better with what Wine needs to provide to Windows code, simplifying things overall. GST is pretty heavy on asynchronous/background processing, which is normally something I'd consider good for media, but if the API you're expected to implement is synchronous then I guess it only adds complexity.
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