Even with a 2nd order boost on SSFS, frequency noise is not yet limiting so much the sensitivity.

Fig. 1: SSFS error signal rescaled by a constant value  in order to match the sensitivity in some point, in the frequency range 1KHz - 3kHz

Fig. 2: coherence between SSFS Err and DARM, in the frequency range 1KHz - 3kHz

Fig. 3: SSFS error signal rescaled by the same value, in the frequency range 100 Hz - 1kHz

Fig. 4: coherence between SSFS Err and DARM, in the frequency range 100 Hz - 1kHz

Below 900 Hz there is a relevant coherence only at 340-350 Hz, and the projection matches the sensitivity using the same scaling factor which works above 1 kHz.