Although we still don't have any scientific tests on the sharpness coming from the new Fotodiox Vizelex Throttle Variable ND Filter 'Lens Adapter', this follow up video answers a few great questions.
Here's another thing they didn't cover which has been a main reason I would consider using the adapter. A fisheye lens on a BlackMagic Pocket Cinema Camera is not so 'fish-eye', and it's one of my go to lenses for wide angle shots. Unfortunately, some ultra wide angle or fisheye lenses (in which the front optic protrudes past the housing) are unable to accept front filters. With the variable ND sitting behind the lens, you can control your image in bright outdoor conditions. I know Rokinon has announced a few great wide angle lenses for Mirrorless cameras, but this situation may be a reason why adapting the larger EF or Nikon mounts would be better.
Hopefully the Micro Four Thirds version comes out soon for the GH4 as I hear it will have a different crop factor than a typical GH3, so i'll really be digging into these ultra wide angle lenses. You can find the (click here)
Fotodiox Vizelex Throttle Variable ND Filter Lens Adapter