I'm sure there are many exceptions to prove the rule, but I'd say the sensor and lens combination puzzle basically looks like this:
1. Most rf wideangle lenses are less telecentric than slr wides; slr rear elements need to clear the mirror, rf wide rear elements generally sit closer to the sensor/film, requiring a more acute light angle to reach the corners
2. Most rf lenses 50mm and longer are as relatively telecentric as slr lenses
3. The M-4/3 sensor requires a highly telecentric light path all the way to the corners
4. The Leica M8/M9 sensor is constructed to require a less-telecentric light path in the corners, in order to accommodate current Leica wideangle rf lenses
So, m-4/3 corner smearing is much more common with wideangle rf lenses than with longer rf lenses, and the Leica M8/M9 has less corner smearing than the m-4/3 sensor. Native m-4/3 wides usually outperform legacy wides in the corners, especially legacy rf wides.
I haven't done any testing, but I'd guess that slr wides should show less corner smearing than rf wides. They don't intrude as far into the body, so their light path should be more telecentric.
Lens light path, sensor design and onboard image-processing algorithms are now connected, and it shouldn't surprise that the $400 Panny lens outperforms the $4,000 Leica lens on the sensor it was designed for.
I don't use legacy rf wides on m-4/3 bodies because I think they'll outperform native m-4/3 lenses. I use them because I can easily manipulate the focus distance and aperture to shoot quickly using hyperfocal focusing. And I use them because I have a couple that are wider than anything offered in native m-4/3 primes. My cameras fire faster with the C/V 12mm & 15mm than with the 20/1.7 or 17/2.8, and both C/V lenses are wider.
I've seen mention of Panasonic announcing new m-4/3 sensor technology that would, IIRC, install the pixels on an aspherical base, allowing tweaking of the corner pixels to minimize corner issues without having to use the complex angled microlenses on Leica's sensor. That would be pretty cool. They could maybe just dial-in the onboard image processing to optimize performance with current native m-4/3 lenses?