Guru3d.com also has a review of the MSI HD6850, which they claim is the first with custom cooling. It looks like the stock numbers are identical to reference, but the cooler is very effective with reducing heat (-10c) but not noise (+1dB).
For overclocking, they mention that:
"We needed to bypass certain limitations though. The Radeon HD 6800 BIOSes are locked. This very 6850 will not go higher then 850 MHz. First up we need to bypass that, in the AfterBurner configuration file set EnableUnofficialOverclocking = 1".
They were able to reach 973mhz with extra voltage, which they called the highest stable OC. That brought the card to +1 fps over the stock 6870 @ only 70c. Just judging by the Afterburner screen capture, it looks like speeds over 1Ghz are supported.
http://www.guru3d.com/article/msi-radeon-6850-review/1