DXO updated their software to include the 180-600. That seems to have fixed the softness issue for me.
Took some shots of my dog which I deleted, but the eye and lashes were sharp.
Took the lens out for a walk today, all shot wide open at 600mm f/6.3. to see how it did. To me it seems a bit soft, but I'm comparing it to my 500PF.
Might have to mess around with the fine tune in the Z8 to see if that helps.