I'm currently playing CPU heavy games such as Planetside 2 and Guildwars 2 WvWvW and thought about upgrading my CPU to a new i5-4670k and ASRock Z87 Extreme4 or Asrock Z87 Fatal1ty Killer if there's any performance gain.
I need a new motherboard for Front 3.0 USB, SLI, Pci-e 3.0 for the future anyway.
I already gave my littlebrother my old HD 7950, but he has an old AMD Phenom II X4 945 with overheating issues so he's stuttering in games such as Counter-Strike: Global Offensive and I thought about giving him my current CPU and Motherboard for his birthday if I upgrade my current rig.
It would help a little bit, but probably not quite as much as you think it might (maybe a few fps gain). It's worth doing anyways in my opinion, since you will be essentially future-proofing, as well as helping out family, and both of those are always good ideas.