Manual overclocking got us all the way to 1460 MHz under The Witcher 3. This represents a respectable 134 MHz boost. We also increased the memory clock rate by 200 MHz without any stability problems over an hour-long session in the same game.
At a glance, that doesn't seem like a lot of headroom. But depending on the game, it could mean anywhere from 7- to 10%-higher frame rates.