Try enabling PBO and finding a setting for the curve optimizer that works for you, each CPU is different but -10/-15 is generally achievable - should reduce temperatures across the board and potentially give you some more performance.
Most definitely - you should always do your own stress testing with your specific CPU (and system) to find out what's stable.
And while all core CO might not be optimal, based on personal experience and what I've seen across multiple enthusiast communities, more often than not you can get an worthwhile improvement to temps/perf with an all core CO.
That being said, there are certainly ways to find and set the best CO values per core, but it will certainly take more effort, stress testing and time.
