OCC has published a review on the Sapphire Radeon HD 7950 3GB Vapor-X

Overclocking the Sapphire HD 7950 3 GB Vapor-X was an interesting endeavor. Using the included Sapphire TriXX software was quite easy and intuitive, so the software really wasn't an issue at all. Using Unigine Heaven 3.0 looping for 30 minutes at a time, I slowly brought up the core clocks; after settling on a maximum core clock, I did the same thing with the memory clocks (while leaving the core clock at the maximum stable value). Usually, this process goes quite smoothly, but for some reason, this card was a bit more picky than most and took me a lot longer to settle in on stable values. Thankfully, after hours of testing, I arrived at the rock solid values of 1100 MHz core clock and 1450 MHz memory clock using 1200 mV and the board power limit at +20. This amounts to an overclock of 150 MHz (16%) on the core and 200 MHz (16%) on the memory over the stock boost clocks. Not too shabby, but I was hoping for more.

  Sapphire Radeon HD 7950 3GB Vapor-X Review