OK, here's the reality. I'll be posting an article on this very soon, but here's the gist of it. Being able to unlock a 6950 to a 6970 is nice and all, but what most people don't realize is that most of the 6970's advantage is purely clock speed. Take Crysis: Warhead (using my own benchmarks) on very high quality @ 1080p (the result for high quality are about the same)...
stock 6970 (880/1375) is 11.5% faster than a stock 6950 (800/1250).
clock for clock stock 6970 (880/1375) is 2% faster than the overclocked 6950 (880/1375).
overclocked 6950 (920/1375) is <1% faster than a stock 6970 (880/1375)
overclocked 6970 (920/1375) is <2% faster than a overclocked 6950 (920/1375)
In case you missed it, unlocking the shaders gives you a whopping 2% performance increase. Overclocking the 6950 gave me a 12% increase, while unlocking and overclocking gave me a 14% increase.
Stay tuned for my full post with benchmarks backing up my claims.