Below is a relative performance chart for all the games benchmarked at 1920x1200 resolution. The link to the review where you can compare individual game benchmarks is as follows:http://www.techpowerup.com/reviews/HIS/HD_7970_X_Turbo/5.html
Using the percentages provide in the below chart the general performance can be derived of the HD 7970 vs the HD 7950. The stock speed of HD 7950 is rated at 73% (compared to the HIS HD 7970 X Turbo). A stock speed HD 7970 is rated at 86%. That means a stock HD 7970 is about 17.8% more powerful than the HD 7950; ([86 / 73] - 1).
The GHz Edition 7970 is 27.4% more powerful than the stock HD 7950; ([93 / 73] - 1).
I'll let you figure out how much more powerful the HIS HD 7970 X Turbo is compared to the stock HD 7950.