SLI of the same card is pushing more frames than a single card of that SLI.
I thought this was obvious.
BTW, I have the same issue with a single 7970. I have my settings such that I get 55 fps 99 percentile minimum on 64 player conquest large.
Even when vsync is enabled the same behavior is presented so no, I don't think it's a cpu bottleneck. I'm pretty sure 99% of the issue is sli is broken on bf4. Watching cpu usage none of the cores are anywhere near maxed.
I can pull a steady 60fps with the setting I've been using. I just wanted to push crazy eye candy with the second card.
I think a big part of the problem is the game not being polished. I experience little problems with my AMD box in single gpu mode but it's pure crap with crossfire so it may just take time for DICE and drivers to be where they need to be.
In regards to COD... I have played every COD title up until now and they have all had problems at launch that took awhile to fix. Not bashing COD but none of those games come close to the scale and complexity that the battlefield series does. 64 players, commander, vehicles, huge maps and a ton of weapons etc make it a lot bigger challenge to tackle than regurgitating COD over and over again. Don't get me wrong, I'm a huge COD fan, but bashing DICE for having buggy releases while ignoring the issues that COD titles have had is one sided.