The Fuji sensor cropped to 24x36 nets 60% of the pixel resolution, or about 31 Mpixel, compared to the M10 @ 24 Mpixel. While this is slightly more resolution, it's a measurable, not perceivable, resolution difference for the most part. The primary advantage for doing this would be to get the GFX50R 16bit output rather than the M10 14bit output in your raw files.
The cost of doing this is that the GFX50R body is substantially larger and heavier than the Leica M10 (see photos attached). And of course how well the Leica lenses will cover the full Fuji format (rather than just the cropped portion of it) depends quite a lot on the lens. Also, you will not have Leica's lens profiles so correcting variations in sensor to lens optical matches will be up to you in post processing.
I use my Leica M and R lenses with the Hasselblad 907x/CFVII 50c camera, alongside native Hasselblad XCD and adapted Hasselblad V system lenses. Some of the Leica lenses perform very well, others not so well. The Hasselblad X and V lenses outperform the Leica lenses on this camera (yes, even cropped to the 24x36mm format) consistently. However, I have some lens options for specific uses (macro, tabletop, etc) using mostly the R lenses that are compelling enough such that I use them, and the Voigtländer 10mm lens on the 907x—even cropped severely to eliminate the hard vignetting—both performs well enough and provides such a radically enormous field of view that it's fun to work with now and then instead of the XCD 21mm, despite the far superior technical performance of the XCD lens.
The Fuji GFX50R would give a use experience a little closer to the M10 than my setup due to its more similar form factor and EVF, and is not constrained to just an electronic shutter like the Hasselblad is. But whether the resulting workflow and quality output is satisfactory for your uses is something that only you as a user could evaluate. If you already own a good range of Leica lenses, and really want a GFX50R, well, for the cost of an adapter you can experiment and discover what works well and what does not. But I wouldn't buy a GFX50R and presume that I was going to get what the camera can provide as its best performance unless I also budgeted to buy at least one or two native lenses made for it.
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