I am not sure that they improved the quality. They have a few new lens designs but the AF problems and the copy variations seem to persist. Their build quality has been gradually improving long before the event you mentioned.
I've owned the total of seven Sigma lenses and used them with 7D, 5D Mk II, and 5D Mk III and the only one that had any problem with AF was a 50 1.4 on the 5D Mk II. It was inconsistent in low light. I sold that lens (which I had bought used) and later bought a brand new copy when I heard it performed better on the Mk III. It's true.
I feel like 90% of the grief that people hear about are from older/cheaper models from Sigma. The 35mm and the 85mm are stunningly sharp. The 17-50 f/2.8 OS was my favorite crop body lens. All the Global Vision lenses offer tuning and upgradeability through the USB dock. You really cannot go wrong with the new lenses.