780 SLI should be more powerful than 7970 GHz Edition CF. If a Radeon HD 7990 gives 82% of the performance of two GTX 780s in SLI at 2560x1600, this means two GTX 780s in SLI outperform the Radeon HD 7990 by 21.95%. Overclocking the Radeon HD 7990 to Radeon HD 7970 GHz edition speeds will narrow the gap, but not eliminate it due to a core clock overclock of 10.5% if using the boost clock of 1050 MHz, and no memory overclock.
Using these same stats, a GTX 780 performs at 64% of two GTX 780s in SLI. So, take the above number, 82, divide it by 64, and you get 28.125% more performance from a Radeon HD 7990 than a GTX 780. Now overclock the Radeon HD 7990 for more performance.
Edit - Oh, ONE GTX 780 vs two Radeon HD 7970 GHz Edition cards.
http://www.techpowerup.com/reviews/NVIDIA/GeForce_GTX_780_SLI/21.htmlEdited by Sonikku13 - 6/15/13 at 4:03pm