Wouldn't a 8700K greatly improve the frames per second on his 1080 Ti compared to an Ryzen 1700? He also does need higher frame rates since his monitor supports it. Plus he can further overclock the 8700K compared to the Ryzen 1700. I would sell the 1700 and go with a 8700K.
With 1440P he wouldn't notice, since the GPU is at the front line then.
Even Ryzen 1600 can handle a 1080Ti fine, CPU bottlenecks are a thing of the past with hexacores being the standard.