Extended iso 50 on Canon 5d mark III video
I'm not sure if you can actually use ISO50 for video on the 5DIII (I can't do it on my 5DII). But either way you'll still ND filters to get where you want to go. No other way around.
There is also diminished DR at ISO50. Not sure how that affects video, but I personally don't find it too limiting when shooting stills and have used that setting on my 5DII when necessary.