Yes, it can happen, sadly. It not uncommon at all. When they 'update' a driver, sometimes a 'fix' they make to improve a newer game will actually cause performance in an older game to drop. I revert to the older drivers when this happens and wait for the next release.
I'll probably end up reverting to older drivers then. It increased performance on SC2, but decreased on every other game I play.
Thanks for the help!