The benchmarks don't lie and show the Sapphire Radeon HD 7950 OverClock edition is only a fraction behind the vanilla Radeon HD 7970.
Thanks to the impressive overclocking potential within the Sapphire card, and the AMD Tahiti core in general, you can easily push the OverClock edition to the same performance levels as the HD 7970.
All of these benchmarks were run at 2560x1600, with the overclocking results taken at the relevant card's maximum achieved overclock.
There's little between them at the very top end of the OC tree.
DirectX 11 tessellation performance
DirectX 11 gaming performance