I must confess that my personal ability to focus is poorer than the J-8 (haven't tried J-3). I've never considered it an issue in practice with a 50mm/f2 , nor any kind of issue at all with a J-12 35/f2.8.
I have a Brian Sweeney adjusted/compounded prewar Contax Sonnar in J-3 mount. No doubt on that one - it's focal length is just off and the suggested issues with close versus long focus, and Contax versus Leica focussing, apply. BUT - I've not had issues there either. The focal shift with aperture is greater, but that's also not personally bothered me. Because I don't really think I'm going to nail exactly correct focus on a moving subject with these lenses, or any others I may have. And if the subject isn't moving, I probably am moving. For precise work on a tripod, I use some other focusing system. Direct vision off the focal plane (view camera or mirrorless digital) would be my preferred.
If you are a street shooter/ happy snapper type photographer, you won't care about the focus issues IMHO. If not, don't get a Sonnar. I think you know what I mean. The wide apertures (over 2.8) are "dreamy" at best in a Sonnar, which adds to the charm but also means that any correctly focused photo is still impacted by the spherical aberrations. There's not a precise focus for all light frequencies - there's a range across the spectrum. I would guess that the right filter would improve (and the wrong deteriorate) focus accuracy.
In short, get the Sonnar of your choice at a good price from your preferred website, and don't get caught up in the precision work. Or if it matters, learn the correction required - coz if you don't have time to correct then your accuracy is the issue, not the lens. And if precision really matters and you want a Sonnar, get a Canon 1.5 (not 1.4), or a Nikkor LTM.
Dante Stella has pages on the focal shift and the above lenses as well.
