Up to 4.0 I don't touch any of those settings. IOH I need to start increasing when I go over 4.4 ghz or so. CPU PLL I haven't touched, even up to 4.9 ghz, although some people have reported decreasing this can increase stability.
IOC I haven't touched, pci frequency comes into play when going over 222 bclk.
Clock skews I also have not personally had success with, although people also report adjusting them can help increase stability.

To run your memory at 2000 mhz will likely need a healthy bump on the QPI vtt (probably 1.34 - 1.4V).