My first Sony CZ24-70mm was very poor at 70mm f/2.8, obviously sharper at one side of the frame. I rejected it and the replacement is fantastic. So it appears that many lens manufacturers have their quality problems and it can be a bit of a lottery.
I now try to purchase from places that will let you return a lens if you think it is poor.
For some reason the first half of my last post was accidentally removed, so here it is again.
My Tamron experience has been mixed.
1. KM28-75mm on APS-C - returned for autofocus fix - better but still soft at 2.8
2. Tamron 28-75mm on A900 - OK at f/4, but again 2.8 mushy.
3. Tamron 17-35mm on A900 OK well stopped down, corners never perfect.
Greg, you got a good one!