I have (finally) looked at the raw voltages for the AS 90 WFS for each quadrant.  The attached screenshot shows the in-lock values from 26 Apr 2016.  The largest value is about 1200 counts, while the smallest is around 130 counts.

The voltages at the input to the ADC are therefore between 80mV and 730mV, assuming 1638.4 cts/V.  Both AS90 WFS are using the full 45dB of analog gain, so the voltages from the demod boards for the AS 90 WFS are between 0.4mV and 4mV.  

I have taken dark noise spectra of each of the quadrants, but since we don't _DQ each quadrant individually, I will have to wait until next lock to get an in-lock comparison.  Screenshot of the dark noise attached also. 

400nV seems like a pretty small number, but it's still a factor of 10 or so more than the noise we expect from the output of the demod boards, so perhaps it's okay, although it's certainly not as awesome as it could be.