Agreed 7nm Zen is the next gen imo but the 12nm ones will be better by some margin at least. but the point being that Intel have had a long time to squeeze that amount of performance from coffee lake, that is why 8% single and 1.2% multi seems quite small, common perception would probably be a bit more perhaps.
Have to remember that % is at the same clocks and i'd presume most 8700k's can do nearly around 1GHZ better than that which gives them a further gain.
Once AMD can get ryzen to closer to those clocks then really i suspect a lot more to change to ryzen.