My CPU is Intel Core i7 6700K, which should be quite decent -or is it? 64Gt RAM and softwares running on fast M.2 NVME SSD. ....
The 6700k is still quite a decent chip. Are you running an overclock?
Until recently I was running an i7 2600k with a 1Ghz overclock using an aio water cooler and the 1080TI . On that system immediately noticed a performance knock with 2017 Lemans as well as the new version of Bathurst from ACtrack reboot. Perhaps this is a result of the new shaders that are used in this version as that seems to be the largest change made from the previous one. Not being a track modder i can only hazzard a guess... But some tweaking is most certainly needed on 2017 to streamline performance.