To answer the question on the Bottleneck. Yes, If you game with a 1080 Ti with a CPU that has less than 8 cores, you will hit a "Speed Limit" in 1080p. In 1080p, the load gets put more on the CPU than the GPU, however, if you went to 1440p, the load would shift to the GPU rather than the CPU allowing possible better frames.
For example, I sold my 2 980 Tis for a 1080 Ti Hybrid. I saw either very little, or less performance than I had with the 2 980 Tis in SLI. The reason for this is because I have a 4790k, and even though I'm at 4.7Ghz, it's still hitting a bottleneck, or rather, "Speed Limit". In other words, the 1080 Ti is too fast of a card for the CPU to keep up.
I decided to test this "Speed Limit" theory. I started at 4.4ghz and used GTAV in DX11 1080p max settings. The frames were at 60-70 outside, with the occasional drop to 56, and inside i was at 90-120. When I increased the cpu to 4.7 Ghz, those numbers went up. to 90+ outside, and 140-160 inside. Now this was with the GPU boosting to ~2000mhz core, when overclocking the GPU, the fps goes back down again. It's almost as if the faster the GPU goes, the less frames you get because your CPU cant keep up.
So i would suggest getting an 8 Core(or more) CPU if you plan to run a 1080 Ti in 1080p or simply move to 1440p and solve the issue.