I'm not sure that the resolution has an impact on the cpu. Number of objects affect the CPU, not the resolution (in the same aspect ratio). At very high resolution the GPU become your bottleneck, then the CPU work less. At low resolution the GPU is very fast, and the CPU can't feed him.
A tiny improvement between PCIe 3.0x8 to 3.0x16, and 3.0x8/8 to 3.0x16/16.
https://www.pugetsystems.com/labs/articles/Titan-X-Performance-PCI-E-3-0-x8-vs-x16-851/
You can experience a decrease of performance notably when the game don't support SLI config..
https://us.hardware.info/reviews/7270/22/nvidia-geforce-gtx-1080-ti-review-incl-sli-faster-card-for-the-same-price-gtx-1080-ti-slinperformance-per-game
In few words, get a Z370 on one 1080Ti. Turn off 2 cores (for DCS) and crank the others. :D