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OP - I get not wanting to make any major sacrifices when you are laying out a grand and a half for the system - but seriously a 1080ti is going to give 30% better VR performance than a 1080.
I would drop to either a 6 core i5 or a Ryzen 5. Honestly 6 cores is enough at the moment, especially unlocked. But you save 30 quid going for the 2600x over the 8600k and with the included wraith cooler I would be confident you can get a good build in your budget. ( There is a pretty awesome bundle deal on OCUK with a 2600 an X370 board, ram and SSD for 354 right now.)
Either way you are going to get a significantly better platform with a 1080ti that will give you a much longer lasting platform. If you go with a 1080 or a Vega 64 you are going to be much more severely tempted by the next release on the GPU side - and it is going to be a much bigger upgrade cost than going Ryzen 7nm in the future.
Its the same cpu with a different name and cap. I'd suggest where you seen benchmark differences the platform or something else was different, or it was within margin of error.