OCC has published a review on the Sapphire HD 7970 6GB Toxic Edition

When it came to overclocking this card I was pleased to see the clock speeds of 1235MHz on the core and 1735MHz on the 6GB of memory. The only other HD 7970 I have tested to reach this level was a factory overclocked and water cooled HD 7970; driving the cooling significantly lower while needing more current than I had to use on the Toxic Edition HD 7970. Sapphire added to the feature set on the Toxic Edition with several means of overclocking the card. The first option is the Lethal Boost button that boosts the clock speeds on the core up to a dynamically boosted 1200MHz and 1600MHz on the 6GB of memory.

  Sapphire HD 7970 6GB Toxic Edition Review