Right now, when I'm playing BF4 my CPU temps go up to 65 degrees using the Hyper 212 Evo. My 4770k is OC'ed to 4.4GHz at Adaptive 1.2v +-0.01. How much will I expect the temps to drop if I upgrade to an AIO water cooler such as the H100i?
Not much if at all. They are almost the same "cooling capacity". You would want to go with an actual water cooling kit for lower temps.
BTW 65C is not bad at all for the Haswell CPU family. They have terrible thermal paste between the actual CPU and the lid. I have a 4670K at 4.4 at 1.23v with an H100i and running prime95 (similar CPU load to intense BF4) for an hour at least and I get 60-65C. All that to say if you're not interested in spending about 200$ for a liquid cooling system, then you are already doing fine temp-wise