There is NO WAY a 2700X will bottleneck a 1080Ti (I notice no-one has asked what else is using PCI-E lanes on the mobo, which could well be a problem with an i7-8700K, or what the monitor resolution is, either).
If I were buying today, I’d get a 2700X and a 1080Ti. But only if my monitor was higher than 1080P.