It probably would make a difference, but it varies a lot by the game.
Also, if you already run at a really high frame rate there's not much point. For example, if your monitor only outputs 60FPS (60Hz monitor) going from 110 to 120FPS won't matter that much. If you're trying to stay above 144Hz by outputting 144FPS+ and even with a 2nd card you drop to 130FPS then overclocking the CPU might make a difference there.
Also, BF4 has CPU issues meaning a faster CPU will benefit. You may be interested in THIS article:
http://battlelog.battlefield.com/bf4/news/view/bf4-mantle-live/
I can't answer your question exactly, but my advice really is stick with a modest overclock of the CPU (4.4GHz at most) to keep things stable. You might gain a couple FPS by going higher but I don't think it would make much difference.
If you're sitting on the fence (i.e. dropping below 144FPS in Crossfire) then a better solution than an excessive overclock is to TWEAK the game settings. A few tweaks can make a big FPS difference with minimal visual difference.