PowerColor HD 7950 PCS+ 3072MB is probably overclocked version of stock Radeon HD 7950. Card has 880MHz set for core clock while memory runs at 1250MHz. Radeon HD 7950 is build on Tahiti Pro gpu, which was made in 28 nanometer process. It features new Graphics Core Next technology. Radeon HD 7950 has 28 compute units, 1792 stream processors and 112 texture processing units. Raster operating units remain at 32. This card has 3072MB of GDDR5 memory by default. It is already known that this card cannot be unlocked to Radeon HD 7970.
Specification from GPU-Z
It is unknown if Radeon HD 7950 has a default clock at 880MHz or this is an overclocked version.
3Dmark 11 Extreme results
DonanimHaber have first verified results of 3Dmark11 Extreme Preset results. As you can see, Radeon HD 7950 beats GeForce GTX 580 in syntethic tests.