How well you can do is determined by your luck in getting a good chip.
As of 5/2016
What percent can get an overclock at a somewhat sane 1.40v Vcore.
I5-6600K
5.0 2%
4.9 11%
4.8 36%
4.7 64%
4.6 88%
Your cooler is fine, you will run out of safe vcore before you run into thermal limits.