I have a setup with an NH-D15S on a pretty good 5820K sample.
With 25C ambients, good case airflow, and a pair of TY-143 fans allowed to ramp all the way to 2500rpm, 4.2GHz core is about the practical thermal limit at 1.2v core and ring (using 4GHz uncore), if I want unconditional stability (meaning it can run AVX2 LINPACK, Prime95, and OCCT without throttling or errors). Temperature in recent versions of LINPACK or 128k in-place FFTs in Prime will approach 90C on this setup. I'm also using the most recent HW-E microcode, which requires a bit more vcore for the same clocks than pre-mitigation microcodes.
With more forgiving loads, I can go a few hundred MHz higher with lower temperatures, but since I consider stability paramount, I haven't really bothered to find the limits of this system under such conditions.
There is no way I'd consider a couple hour long render in Blender a useful stress test on for anything more than a pure benching or gaming build.
...rightful liberty is unobstructed action according to our will within limits drawn around us by the equal rights of others. I do not add 'within the limits of the law,' because law is often but the tyrant's will, and always so when it violates the right of an individual. -- Thomas Jefferson