The .64x Metabones Speedbooster NF to BMCC adapter, an improvement over the native adapter?
The older/original/"Ultra" speedboosters, and some of the newer ones (e.g. all of the ones for Fuji X bodies) are the .71x magnification. The newer/improved/"XL" version/design speedboosters are the .64x versions.
From a physics standpoint, the .64x version has a wider field of view, and better brightness than the .71x version. Also, you can achieve an incredible f/0.8 by pairing a 0.64x Speed Booster XL with an f/1.2 lens.
A couple years ago Brian Caldwell (the guy who designed the speedbooster optics) said on Nov 18th 2020 on DP Review forum-
"Bear in mind, the .64x Speed Booster XL is a considerable improvement over the original .71x Speed Booster for M43."I have owned both versions. Since reading Brian's comment, I now only own 4 of the .64x version and none of the .71 version.
Also, I have owned the NF - MFT metabones speed booster. I can not recall if it's .71x or .64x; but I did think the image quality of the NF to BMCC version was better. The rear of the NF/BMCC is much closer to the sensor, maybe that's why (so close you can only use it in the silent/electronic shutter mode).