I bought a Nikon to MFT Metabones Speedbooster for the GH2, which served me quite well for a year, particularly with the remarkably good Sigma 18-35. The crop factor was 1.42x, which was just about perfect with the Sigma: any wider would have caused vignetting, as evidenced in the fact that I could only use it with the lens hood off.

Now that I'm using a GH4, I am still using this setup, but in UHD mode I'm getting a 1.7x crop, which is a little annoying to me. My Sigma 18-35 suddenly doesn't feel quite wide enough on the wide end for a walk-about lens, and I struggle to get good subject isolation in situations where it never used to be a problem.

Another annoyance is not having autofocus, or the possibility of image stabilization if I buy a VR Nikon lens in the future, due to the Nikon Speedboosters not having electronic contacts. I don't use autofocus in video, but I'd feel a lot more comfortable shooting stills if I had autofocus. I'd also feel more comfortable shooting video and stills beyond 85mm, if I had image stabilized lenses that are supported as such on my GH4.

Finally, around here everyone and their mom is using Canon. I'm mostly happy with my modest lens collection, but there are definitely times when I'd love to be able to throw someones' Canon 70-200mm f2.8 on my GH4.

To me, it seems like the best option at the moment is the Canon EF to BMCC-MFT Speedbooster. This has a 0.64x reduction, yielding a crop of 1.53x on the GH4 in UHD mode, which is damn near Super 35 -- perfect, in other words. It also inexplicably has electronic contacts (surprising, because the BMCC-MFT doesn't). This would mean that I get autofocus and image stabilization as an added benefit.

Finally, as far as I know, I could use a basic Nikon to Canon adapter ring to mount old Nikon AI/AI-s lenses (as well as newer Nikon glass with manual aperture rings, like my Rokinon Cine lenses) onto a Canon to BMCC-MFT Speedbooster, opening up the way for even more lens choices with a single Speedbooster.

I've read rumors about a GH4 specific Speedbooster. I also would happily spend the extra cash on an "Ultra" variant Speedbooster, if it yielded improve image quality. These are the main considerations that are holding me back from exchanging my Nikon-mount Sigma 18-35 for a Canon EF mount copy, and exchanging my Nikon to MFT Speedbooster for a Canon EF to BMCC Speedbooster.

Any thoughts on these questions would be very appreciated. I'm a little reluctant to sell my Sigma and Speedbooster, crippling my lens options for at least a week while I wait for Canon replacements to ship in... but overall, it seems like the best idea.