The card seems to overclock pretty well, a Chinese overclocker used it to break the 3DMark Vantage record for the Radeon HD 6850:
Shortly after its introduction, the ColorFire team led by overclocker Carl from China was able to break a 3DMark Vantage record for the Radeon HD 6850. The overclocker achieved P27751 points, some 3810 points ahead of the previous record-holder. The card sustained overclocked speeds of 1500 MHz core and 1275 MHz (5100 MHz GDDR5 effective) memory, and backed by an Intel Core i7-980X clocked at 5.7 GHz, with the entire setup being cooled by liquid nitrogen.
Source: TPU