Look at this comparision, clock-to-clock GK104 and Tahiti XT chips. Where is better HD 7970 a where GTX 680? - Some numbers are really interesting ... enjoy!
Setup: Core i7-3930K @4.7 GH, Asus Rampage IV Extreme, 8 GB DDR3-2400 MHz Kingston, Antec HP-1200W. Drivers Catalyst 12.3 + GeForce 300.10.
3D Mark Vantage + 3D Mark 11 - Extreme Preset, 1920x1200 (1080)
Heaven 3.0 - 2560x1600 Tesselation Disabled, 4xAF + 4xAA, 1920x1200 Tesselation Extreme 16xAF + 8xAA
Crysis Warhead - 2560x1600 Enthusiast, DX10, 4xAA
PS. Radeon has 384-bit membus, on 4 GHz is bandwidth the same as GeForce 6 GHz on 256-bit membus. In table is 4 GHz for equal mem bandwith comparision, with this bandwidth is Tahiti shit, worst then GK104. But with huge mem bandwidth 6 GHz (on 384-bit) is far better then GK104. But this is not default clock ... In effectivity point of view is GK104 far better then AMD chip. Smaller, lower power consumption with the same performance.