There are already some online comparisons of Kaby Lake vs. Sky Lake. (Try YouTube or Google for reviews/tests.)
The performance bump 'clock for clock' is possibly about 15% in GAMING if I recall correctly. Clock speeds/overclocking seem a little higher than Sky Lake.
If you have a Z170 motherboard, assuming the manufacturer releases a bios update, then you will be able to use Kaby Lake. If you buy Z270 (say) then it'll work out of the box.
If you already own an i7 6700K I wouldn't bother rushing out to buy an i7 7700K though. Not worth the extra ca$h right now imo. But if my 6700K runs out of steam in 2/3 years in certain games, well I might be tempted in the future...
Watch this...
https://www.youtube.com/watch?v=ys9Gtc4oC2A&t=60s