Just one remark, currently there exist noe Metabones Speedbooster for Canon EOS -> M43 (GH4).
Form what I hear, Metabones are struggling with the electronics to make AF work.
Some cheap focallength reducers "speedbooster-copies" are available on ebay, though fully manual, and optics are probably not as good as Metabones.
Does the A7S support only 720p in S35 mode? Or also 4K and 1080p ?
Ole