Oh man, I love old X99 threads!
FWIW, I still have a 5960X running on an AsRock OC Formula as my media PC and I run that CPU at 4.63Ghz w/ 125Mhz BCLK and it is 100% stable only using 1.78V Input Voltage and a VCore of 1.32V. VCCIO is 1.05V, SA was around 1.10~1.15V effective if I recall (id have to look again), Ram is running at 3000Mhz Quad Channel 32GB CAS 13 with 1.31V on the sticks (they are B-Die).
That 5960X I have was a golden sample or something... lol. It was a J-Batch.
I'm still rocking my I7 5960X as well
But I think I have the golden of the golden samples
I've been running mine @ 4.5ghz 1.17V Vcore, 1.8V Vccin (tops at 1.84) and 1.05V Vccio for a few years now, completely stable... I can even run it at 4.8Ghz @ 1.3V, but I only did for some benching... at 4.9Ghz I need around 1.38V Vcore and things get hot, so I've done only a benchmark to see how it was, didn't risk benchmarking at 5ghz, but the system did turn on.
My MB is a MSI X99A XPOWER AC
I also have an I7 5820K @ 4.5ghz - 1.25V and an I7 3970X @ 4.5Ghz - 1.28V