A overclocked 980 is no where near as fast as overclocked 7950s I think you will find.
Check the firestrike thread:
http://forums.overclockers.co.uk/showthread.php?t=18485414
The only way one 980 will beat two 7950s is with liquid nitrogen
Here is the top water cooled 980 overclocked
Heres my 7950s on Air overclocked
Look at the graphics scores btw, talking about gpus here not the cpu or combined scores.. a 980 averages out at 15500 by looking at peoples results, two 7950s average out at 16750 graphics score.
So like I say only way to beat the graphics score of two 7950s overclocked is to overclock a 980 with liquid nitrogen etc, but ofc in gaming, it depends on how well the game scales with crossfire, but raw performance a 980 cannot beat 2x 7950s.