I get 120-140 fps (mins 80fps) in BF4 with a 3750k (4.7Ghz) running 2x7950's.
Contrary to what people say I think it is total sheet that an i7 will give a large fps improvement in nearly all games, my brother has a 6 core, 12 thread 3830k and the same gpu's as me, that basically runs the same fps or slightly less because he can only clock to 4.4Ghz stable.
If you cant run a 4.5Ghz+ overclock then there maybe a cpu bottleneck on ivy/haswell i5's using 2 cards, but I dont think I would change my i5 for an i7 as my friends on BF4 often get lower frames than me with i7's running 2x670's, 2x680's etc. I'm still very happy using the i5 ivy bridge for gaming, dont waste your money if you really dont need to. (for Gaming at least).
Edit,
I have found the i7 with threading on seems to lower the peak cpu usage by around 15% in battlefield 4 with everything max'ed out.