That's a nice result, well played
Depends, especially in terms of gaming because that is a realtime workload so every 100 mhz is a few frames and with higher clocks, a higher RAM freq also pays off again. But, that isn't much different than it'd be on a 9600K. Put differently, if you get these K CPUs and DON'T overclock them, why even bother / waste the money? There are far better offerings in that case, like most Ryzen CPUs that run lower temp and auto OC out of the box with XFR and come with a heatsink.
About chip longevity; you can easily run your 9600K at 1.3~1.4V 24/7 and it'd last 8-10 years no issue. Don't worry too much and get those damn tools out already!
Lifespan decreases happen at very high voltages, which I'd say is beyond 1.4V. 1.52 might be a safe max but its really quite high, also for the board, traces etc.