That temperature is a kinda high if you were just gaming and it reaches above 70C. Normally it should only reach 70C or above at 100% load, which I doubt your game could.
Have you tried running prime95 version 26.6 small FFT test (not blend test) on that overclock?
If that overclock can sustain the test for at least 6 hours and temperature stays at 75C~80C, then I say you have a solid overclock with appropriate Vcore.
If it fails to pass that test then you need to increase Vcore if you still wanna stay at 5.2 Ghz, OR, keep the same Vcore and decrease the overclock to 5.1 Ghz.