It seems that this could really change one's workflow. Recording in enhanced 1080 would save an enormous amount of hard drive space and in some ways simplify things.
The GH4 essentially does this in camera, but of course you lose the low light capabilities of the A7s. I wish I could see more tests.
I'm also wondering if the Atomos Shogun or any similar recorder could do this in a single unit. Could it take a 4K input and downscale to 1080 and record. If this would increase the detail while keeping file sizes low, it would be useful.