It's a known that the 10.11 drivers broke stuff fixed in the 10.9/10.10 drivers tho the fixes were supposedly for games other than Wow.
I'm switching to NV as I have just bought 2xGTX460's mostly because I have found the ATI drivers to be so flaky and the dev's should work for bethesda..they would be right at home.
I'm running 10.11 drivers atm and they seem to work fine with Wow (running 1920x1200 and ultra settings but BC2 is a whole nother thing.
I would think you need to get rid of all your old drivers and do a fresh install of 10.9 drivers. You dont need to install in safe mode tho..normal is quite fine
And as someone above said if your drivers work fine then don't update as it can open a new can of bugs