No especially if you don't overclock, I would consider getting some cooling and overclocking your current CPU. The price for the new coffe lake is way to high and you will need a new motherboard. Not worth it.
is 6700k still good ? I think that's the answer I am looking for.
IF i upgrade my GPU say 1080Ti (lets say I have 4k monitor for now), will my CPU bottleneck My GPU ?
At that point your GPU will be doing all the work. Your 6700k will be fine with that resolution as long as your GPU is. Sure a 8700k will be better but a better GPU will give you the most performance.