Based on the benchmarks in the following review of two OC'ed HIS Radeon HD 7850 IceQ TurboX 2GB Crossfire vs. a stock speed Diamond Radeon HD 7970, the average performance increase is approximately 31% when looking at the 1920x1200 benchmarks.
http://www.tweaktown.com/reviews/4798/his_radeon_hd_7850_iceq_x_turbox_2gb_in_crossfire_video_card_review/index6.html
The biggest performance increase is in HAWX 2 which is just above 56%.
The lowest performance increase is in Mafia 2 which is barely above 10%.
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As I stated, the average performance increase is about 31% above the stock speed Radeon HD 7970. Looking at the performance chart below of relative performance at 1920x1200 resolution, a stock speed Radeon HD 7970 has 71% the performance of the Asus GTX 780 DC II OC 3GB card.
With an average 31% increase in performance (71% x 131% or 71% x 1.31), that means the two OC'ed Radeon HD 7850 XFired together will have about 93% the performance of the OC'ed Asus GTX 780. That makes it about equal to a stock speed GTX 780.
Source: http://www.techpowerup.com/reviews/ASUS/GeForce_GTX_780_Direct_Cu_II_OC/26.html