Attachment #1  Measured error signal spectrum with the Oplev loop disabled, measured at the IN1 input for ITMY. The yaxis calibration into urad/rtHz may not be exact (I don't know when this was last calibrated).
From this measurement, I've attempted to disentangle what is the seismic noise contribution to the measured plant output.
 To do so, I first modelled the plant as a pair of complex poles @0.95 Hz, Q=3. This gave the best agreement with measurement by eye, I didn't try and optimize this too carefully.
 Next, I assumed all the noise between DC10Hz comes from only seismic disturbance. So dividing the measured PSD by the plant transfer function gives the spectrum of the seismic disturbance. I further assumed this to be flat, and so I averaged it between DC10Hz.
 This will be a first seismic noise model to the loop shape optimizer. I can probably get a better model using the GWINC calculations but for a start, this should be good enough.
It remains to characterize various other noise sources.
Quote: 
Before the CDS went down, I had taken error signal spectra for the ITMs. I will update this elog tomorrow with these measurements, as well as some noise estimates, to get started.

I have also confirmed that the "QPD" Simulink block, which is what is used for Oplevs, does indeed have the PIT and YAW outputs normalized by the SUM (see Attachment #2). This was not clear to me from the MEDM screen.
GV 30 Jul 5pm: I've included in Attachment #3 the block diagram of the general linear feedback topology, along with the specific "disturbances" and "noises" w.r.t. the Oplev loop. The measured (open loop) error signal spectrum of Attachment #1 (call it y) is given by:
If it turns out that one (or more) term(s) in each of the summations above dominates in all frequency bands of interest, then I guess we can drop the others. An elog with a first pass at a mathematical formulation of the costfunction for controller optimization to follow shortly. 