From the looks of your results you're fine the way you have it for stock speeds, the chip is doing OK and you reported it getting to about 40c while gaming, that's good.
While I do have a 7700K coming my way I've yet to actually run one myself, maybe another that's already running one can tell you what voltage is good for 5.0 on average - Just know that no two chips will act the same, some wanting more voltage than others to get the same clocks from it.
Best way to find out on your own if you must is simply to start clocking it up with no change in voltage and see how far it goes. Once it quits booting then bump the voltage up a little, then go again until it quits booting again. It's a lather, rinse, repeat process as you go until you get the clockspeed you want. Note that temps UNDER LOAD, not idle are what you need to worry about once stability testing is being done. Get to the speed you need, then set the voltage to what's needed for stability (And no more) and once you have all that squared away, you're done.
Don't be suprised if for some reason you can't run it at 5.0 period, some chips just don't want to run that high and if your chip hasn't been delidded yet it will probrably run warm to hot anyway. However with the chip's stock speed it should have NO probs getting 5.0 I'd think.
Most stock Intel chips nowadays will run very warm to just hot due to the crappy TIM job under the lid once you start cranking the MHz from them.
You'd have to address that if you must get 5.0 from it but temps are getting too high.