From Anandtech Benchmark comparing 7970 Ghz Crossfire vs 7970 GHz:
http://www.anandtech.com/bench/product/769?vs=768
Dirt Showdown - 1920x1080 - Ultra Quality + 4x MSAA + Adv. Lighting 158.1 fps vs 91.3 fps (73.2% Scaling)
Total War: Shogun 2 - 1920x1080 - Ultra Quality 112.2 fps vs 62.3 fps (80% Scaling)
Hitman Aboslution - 1920x1080 - Ultra + 4x MSAA 89.3 fps vs 58.6 fps (52.4% Scaling)
Sleeping Dogs - 1920x1080 - Ultra Quality + High AA 129.4 fps vs 97 fps (33.4% Scaling)
Crysis Warhead - 1920x1080 - Ultra Quality + 4x MSAA 69.8 fps vs 39.6 fps (76.3% Scaling)
Farcry 3 - 1920x1080 - Ultra Quality + 4x MSAA + Enh. AtoC 77.3 fps vs 43.2 fps (78.9% Scaling)
Battlefield 3 - 1920x1080 - Ultra Quality + 4x MSAA 144.6 fps vs 74.4 fps (94% Scaling)
Civilization V - 1920x1080 - Maximum Quality + 4x MSAA 113 fps vs 94.8 fps (19.2% Scaling)
Bioshock Infinite - 1920x1080 - Ultra Quality + DDoF 132.9 fps vs 74.4 fps (78.6% Scaling)
Crysis 3 - 1920x1080 - Very High Quality + FXAA 75 fps vs 40.4 fps (85.6% Scaling)
Depending on which games you play, you can see up to 94% scaling from crossfire 7970 Ghz in 1080p. Whether or not it is worth upgrading from 580 SLI is up to you, but you can sell the 580 SLI for $150 - $180 each and recoup majority of the cost. I would say that is a pretty good upgrade.