It depends on the application/game you are running as to whether or not they will run at the overclocked speed. Some benchmarks may run while some games may not. So it's up to you and the programs you run to determine your max operating frequency. I usually settle for just under a "maximum" frequency. When I run my GTX 1070 overclocked I run it at +400MHz mem because I found it crashes the games I play the more I go up from there.
I tried mining crypto with it and it can do +600MHz mem or more doing that, but it will crash playing a game.