My 7700K gets remarkably hot. I did a delid, replaced the thermal paste with a liquid metal one, and placed the heatspreader back on the cpu with a much smaller gap. I'm running the CPU at 4.9 GHz on all cores and -40 mV Vcore offset (still below the default cpu voltage). Cooling is achieved with a fully custom watercooling loop with two radiators (2x 120 mm and 3x 140 mm fans), and the water temperature is rarely above 35 ?C. However, even when just gaming, the CPU already runs at 50 - 60 ?C. Under full sustained load on all cores it goes up to 70 ?C. With watercooling! Those temps are insane. I don't even know how you're supposed to cool Kaby Lake with air.
Previously I had a 2600K, also OC'ed to hell (+1 GHz on all cores compared to stock), with a positive Vcore offset and that thing never went higher than 60 ?C with watercooling, even under full load. In games it usually leveled out at 40-50 ?C. Kaby Lake seems to run at least 10 ?C hotter, even with the replaced thermal interface and reduced gap. Nevertheless, it's a fine piece of CPU, but I don't see too much of a difference between it and my previous 2600K (mind you, 4.9 GHz compared to 4.5 GHz is not that much of a jump anyway, and the architecture isn't sooo much faster either, so I wasn't expecting too much in the first place).
Reasons for replacing the 2600K: Boredom, mostly.