I will attach a document which describes how the noise affect the wiener filter and the noise cancellation ratio.
And I re-estimate the SN ratio in the microphone (but still rough):
The yellow line is modeled signal level, and cyan line is modeled noise level.
Then, the estimated filtered residual noise is:
The noise is already subtracted enough below 80 Hz even though there is still coherence.
Above 300 Hz, the residual error is limited by other noise than acoustic noise since there is no coherence.
I am not sure about the region between 100-300 Hz, but I guess that we cannot subtract the acoustic noise because primary noise (see the document), such as a peak at 180 Hz, is so high.