Overclockers Club published a review on the Intel Core I7 6950X Extreme Edition Broadwell-E Overclocking A quote from the article:

The single biggest hurdle when running higher voltages and clock speeds is thermal management. A nice air cooler or even some of the nicer AIO kits are going to be fully overwhelmed with the thermal load displaced by the Core i7 6950X when overclocked and over volted. I had to resort to a full on water cooling setup to reach the observed 4.34GHz and 4.28GHz speeds I ran in my testing. Usually the hefty 240mm radiator I use for overclocking processors is sufficient to handle the typical load. Not so with this beast. I had to back up and put in a second radiator to handle the load discharged into the liquid. By doing so, I was able to keep the processor from throttling and bouncing off the 99 °C max temperature before the chip could throttle. Overall, most of your usual overclocking tools and methods that we have worked with over the past few generations of Intel processors work well with this generation of Intel's Extreme series chips. The new flexibility adds some wrinkles, but nothing that cannot be easily overcome. The Intel Core i7 6950X Broadwell-E processor is one hell of a processor.

 Intel Core I7 6950X Extreme Edition Broadwell-E Overclocking Review @ OCC