The distortion is shocking. While I hate the idea of an adapter, no IQ improvement is worth the field curvature with this lens. All of that extra sharpness will get lost in post.
Going by Bryan's results, there's no actual IQ improvement anyway. Sharpness is a dead-tie, each one varying
slightly in different areas of the frame at different focal lengths, and fringing on the new lens is worse even aftrer corrections. Vignetting is also slightly worse, too, which usually corrects fine at low ISO but if you're shooting at medium or higher ISOs then that extra correction is going to bring out more noise; technically not a lens IQ problem, but an overall image IQ problem all the same.
Note that I'm not positive that's the case – maybe this lens on an R5 will yield a 39.5 MP image at 14mm, a 41 MP image at 16mm, etc. I doubt it, but I really don't know. Since Canon does a similar 'trick' with the RF 24-240, it would be occurring there as well and I haven't heard anyone mention that the 24mm images from the superzoom are lower resolution.
This sort of correction isn't just cropping away the edges and presenting a lower res file, no. Some parts of the image are being squashed down while others are being stretched out. The final image resolution out of the camera is always the same.
What
is being reduced is the
optical resolution, since it is not physically possible for distortion to be corrected without lowering optical resolution, even if you keep the file resolution the same. So the file's pixel count may still be the full 45mp, but the optical resolution may only be 40mp, stretched out to 45mp. (And actually optical resolution is never that high to begin with, so really it's probably more like 30mp of optical resolution stretched out over a 45mp file.)
The 24-240mm is an interesting case because the non-linear distortion means the center of the image really doesn't need much correcting if you're focused more than a few feet form the lens. If you don't want a 3:2 file and you focused, say, six or seven feet away or more, then you can just crop the sides a little bit and you have a roughly 22mm shot which is much sharper than the corrected 24mm; Canon seem to
over-correct the 24mm quite significantly and bring it to more like 25mm, possibly to account for the extra distortion the lens suffers from once you bring it close to minimum focus. It could be that this 14-35 is the same kind of case and perhaps a higher optical resolution could be retrieved from it if a squarer crop is acceptable and depending on the focus distance.
Still, I argue these are not things that anyone paying £1750 should have to be thinking about, anyway. The 24-240 gets away with it because it's a plastic superzoom made to be an hobbyist's jack-of-all-trades, and priced accordingly. This L lens is
not advertised or sold as such and nobody dropping this kind of premium—again, 80% higher than the EF equivalent new, let alone third-parties—should be having to have these conversations.