I think you were told wrong.
The NH-U9B is similar to the current NH-U9S.
Noctua has a tdp chart for its coolers.http://noctua.at/en/tdp-guide
Your I5-7600K is a 91w processor. More with overclocking.
The chart would indicate low overclocking potential.
I would return the cooler in favor of one of the new s variants if I could.
I do perfectly fine with a well binned I5-7600K@4.9 using a NH-U12s.
Your results will vary, depending on your luck in getting a good chip.
Here is what you might expect:
As of 6/19/17
What percent of samples can get an overclock
at a vcore around 1.4v.
5.3 samples exist, unknown % of occurrence