It is important to note that all HD 6990s come with two BIOS settings that the user can switch between using a switch located near the CrossFire connector. Here's a picture from TechPowerUp that shows what the switch looks like:
All the sites listed in the chart below (except for TechPowerUp) overclocked their cards with the BIOS switch at position 1 which gives the card a clock speed of 880 at 1.175 volts instead of 830 at 1.12 volts. This is probably why TechPowerUp got a lousy core speed even though they managed to get the highest memory overclock. Major props to HardwareHeaven and Neoseeker for reaching the coveted 1Ghz mark!