Lol! I've just found out why Windows would no longer boot with Internal PLL Overvoltage Disabled at those x50 + multipliers...
Turned out it was the CPU Spread Spectrum setting causing this. I had disabled it, and when I put it back to AUTO it would boot again at x50 multi and PLL feature disabled
Update: This also may have caused my CPU needing a tad more Vcore (I thought it had degraded) since it seems good so far with my old 5GHz preset. I never would have thought that Disabling CPU Spread Spectrum could have caused all this.
"CPU Spread Spectrum: Modulates the processor clock to reduce radiated noise emissions. Disable if overclocking, as clock modulation will increase instability.
Seems like better OCing with it enabled