I picked this up and have been testing. The assembly looks a bit frankensteinish but seems to serve the purpose. I agree with CatAreGods comment on this maybe being unnecessary under some circumstances. I am pretty happy with it from the standpoint that it lets me balance the entire camera/lens/teleconverter assembly much better over the tripod mount. That will come in handy with windy or other challenging conditions.
I'm going to throw in my two cents here. I have the Fuji GFX 50r and the 50MM pancake lens. This is a great street and general shooting 1-lens kit. As to the difference between FF and MF, I also agree with Jim, I see a difference in the color and resolution on screen (I have yet to print from my Fuji yet). A friend refers to this characteristic as the "special sauce," the combination of Fuji's sensor and lenses. Yes, I have FF Canon R6, the files are good, but there is a visual difference that to me looks richer with the 50r. The shot of Mano was taken with the 50MM lens on the 50r.