There are a few different ways to overclock on the X99P-SLI. The easiest way would be through the EasyTune software. You can select Auto Tuning and it will try and find the best way to overclock your system. Running the Auto Tuning feature it brought us up to 4.10 GHz.
You can also select one of the 5 Quick Boost overclocks.
If you head into the BIOS there is the CPU upgrade option in the M.I.T. that gives us a list of different overclocks.
Finally we can manually overclock in the BIOS itself. I decided to do this to reach the highest possible overclock. After some testing I was able to reach a max stable overclock of 4.3 GHz at 1.35V. I was a little disappointed as all of the other X99 motherboards I have tested made it up to 4.4 GHz stable.