My bottom line conclusion is that if you're invested in quality rangefinder glass, you'll get the best (most faithful) across frame performance with 35mm and wider, as well as some 50mm lenses, on a digital M. And of those options, the M240 is the best (if you want color).
I've shot a number of RF lenses on NEX as well as the a7 and a7R cameras. I've tried two adapters - a Novoflex and a Metabones. The Metabones had an extremely tight fit to the lens and wouldn't allow infinity focus. After realizing these two points, I stopped using it. The Novoflex seems OK, though I can't rule it out as a cause for some unevenness of sharpness across the frame with specific lenses compared to how those lenses look on a digital M.
The problem with most WA RF lenses is they have fairly short exit pupils, relative to SLR designs. This results in a more oblique angle at which light rays reach the sensor at the image periphery and are refracted through the sensor toppings in a manner that degrades image quality. The thicker the sensor toppings, the worse the degradation. Sorry if I'm repeating what you already know, but ideally you want to find a sensor with the thinnest possible toppings for WA RF lenses, especially if it's a full frame sensor. Currently, only Leica strives for this.
How bad the image degradation is perceived to be also depends significantly on the type of photography. I.e. brick walls/architecture/infinity landscapes tend to be more critical than portraits and anything else where there is great subject background separation and the subject relatively centrally positioned.
