The performance for Witcher 3 is unknown as of now.
It is CD Projekt Red, so it will be great and greatly optimised. I'm psyched.
"Bottlenecking". I swear, that is the most overused term when talking about CPUs. It appears everywhere. It's almost like Godwin's Law, except for CPUs.
Barely any fairly modern CPU will bottleneck a 970. PCIE 2.0 will not bottleneck it, i3 will not bottleneck it, even Pentium in some cases will not.
You could go with literally any Intel CPU.
I suggest a 4790k. Witcher 3 will probably have some support for Hyperthreading at the least. Besides, the thing's a tank:
http://www.amazon.com/Intel-i7-4790K-Processor-Cache-BX80646I74790K/dp/B00KPRWAX8