Thank you!
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I ordered the Hasselblad 907x Special Edition set shortly after they were announced in mid-Summer 2019. The one native lens I ordered with it is the XCD 21mm, to net an "all digital Hasselblad SWC" in square crop, and I will buy another one or two X system lenses for it (already have the 45P on order...).
But ... I have the Fotodiox Pro adapters for both M and R mounts, and I wondered yesterday what a couple of those lenses might perform like on the Hassy's 33x44 mm sensor.
So I pulled out two of my extreme wides for the R system and then both my ultra-ultra wide 10mm for the M as well as the 43mm wide-normal. Here are some results.
In each of these panels, the top row is the XCD 21mm as reference. The left column is the full frame rendering; the right column is a square crop to the maximum image area where no vignetting occurs. All exposures at ISO 800 and f/8, shutter speed jiggered around to get commensurate exposure per frame, and processed in LR to make exposures close as I can (quickly). Subject to camera distance in all cases is ~4.2 feet.
R Mount - I tried the Elmarit-R 19mm f/2.8 v1 and the Super-Elmar-R 15mm f/3.5. Given that the 19mm is only 2mm shorter than the XCD 21mm anyway, and vignettes, there's really no point to using it in preference to the 21mm. The 15mm nets a useful additional amount of FoV. Both lenses, in their non-vignetted area of coverage, render very nicely.

M Mount - The two lenses I tried in this test are radically different, both from each other and from the XCD 21mm. And they're interesting: The Pentax-L 43mm, being essentially an SLR lens specially adapted to LTM/M-mount, vignettes only at the corners of the full frame. Cropped vertically, it's perfect. The Voigtländer HyperWide 10mm, an ultra-ultra-wide angle that just barely covers the format even on FF, shows this wonderful view that looks like you're looking out of the mouth of some toothy beast (yup, those are the little stubby bits of the rudimentary lens hood built into it...) in the full frame image. Cropped*square down to just within the hard vignetting, it shows substantially more FoV to work with than the XCD 21mm still. And at full resolution, it's a pretty darn good performer throughout. An impressive lens.
🙂

Higher resolution versions of these comparison grids are available by clicking through to Flickr.com on them.
I've also used the Macro-Elmarit-R 60mm, the Summicron-M 50mm, and the Elmar-R 180mm + 2x Extender-R on this camera. All three have done well, the 50 and 60 even on the full 33x44mm format, the long tele best when cropped to square.
enjoy!
G