OK, I think I have a good one: the one I sent for repair, and which has been sent to Japan because they couldn't fix it in the UK, has been replaced by Sigma UK. It isn't clear whether or not they intend me to keep this copy or whether it is a loaner until the Japan-bound copy gets fixed but the one in my sweaty hand now is the first one I have had with no asymmetry that I can find in an hour of shooting.
It does, however, produce shots that are soft on both sides rather than just on one: but that is a good thing I think and pretty much what I would expect.
Let me clarify: the lens clearly has a field of focus that curves forward towards the edges. That is not unusual, and the copies I had that were sharp on one side were only so at the expense of extra softness on the other side because the asymmetry was skewing this field of focus. So what I have clearly demonstrated to my satisfaction is that the copy I have now, at F5.6 CAN get sharp edges too, but only when focussed a little further back than my +20 AFFT can allow.
I think that my body, when it went for the left side focus fix, has ended up with a general plus requirement - my other lenses pretty much all need a plus of some amount but none as much as 20, whereas every copy (now five!) of the siggy I have had has needed 18 to 20, usually 20.
So as soon as the USB dock is available for Nikon fit, I will adjust the lens so that it needs less plus fine tune, and then jig it so that it focusses as far back as possible without losing the plot at F1.4... I suspect that with my current setup, a +23 to +25 might achieve that, were it possible.
The good news is that I really do like this lens. Wide open, it's like a well-behaved F1 Noctilux in some ways, in terms of its look, but has a less complex field curvature (albeit being a stop slower) and better control of aberrations. Then, when focussed correctly and stopped down a bit, it gets to be a much more technical lens.
I am feeling good about this one!