I'm still in the process of it but I was looking at the what the i5 2500k @ 4.5Ghz is in comparison to my i7 6700k that I have now (going to 4.5Ghz for better speed comparisons).
Though really even a 1080 shouldn't bottleneck the CPU but doesn't mean the performance of the game might not increase due to having a newer CPU.
I think bottleneck implies that one or the other cannot physically run at 100% due to a limitation of the other.
Yes, I agree, but for gaming, if the GPU is able to be pegged at 100%, the CPU is fast enough in general to not 'bottleneck' the GPU. It's not actually REDUCING the performance of the GPU. It would only be 'bottlenecking' if the GPU was sitting at say 70% with a 100% CPU load. If that makes sense? Of course there will always be one that is 'limited', but for gaming (the purpose of the OPs question), I don't think it'll be a 'bottleneck' in that it would stop the GPU from performing.