I have 2 680s and a 3770k at 4.2ghz and you're managing the same fps as me, both GPUs are running at above 90% majority of the time, I seriously wasted money buying a 2nd 680.
ahh i only have a i7 2600 not k series so its locked at 3.8ghz turbo but i just fixed the problem. I turned up the texture quality from high to very high and that made my fps shoot through the roof. Its really strange that it works that way but now im getting 90+ on mp maps with vsync on.