At stock speeds the difference between the AMD Radeon HD 7950 and HD 7970 is actually quite pronounced, sometimes by as much as 25%.
That's a pretty reasonable performance difference between your top two GPUs, but it's not actually much to do with what's been chopped out of the GPU architecturally, more to do with the drop in clockspeed.
Once things are evened out in the core clock, through overclocking, there is suddenly almost nothing between the two AMD HD 7000 series cards.
The CrossFire performance is impressive for the AMD Radeon HD 7950 too, offering similar frame rates to the pricier cards, and the savings over the HD 7970 are multiplied in multi-GPU arrays.
DirectX 11 tessellation performance
DirectX 11 gaming performance
DirectX 10 gaming performance