This is a good first step, but there are two points:
1) You can NEVER use the SR785 for the FSS loop gain measurement since it doesn't go above the UGF. Use the RF network analyzer and do multiple sweeps to get the resolution using the Mott GPIB code.
2) This seems like its not good enough in the 10-100 kHz band.
3) We need to see how much phase is being lost because of the low modulation frequency and the high Q of the RFPD. Where is the plot comparing the servo model and the measurement?