Conclusion:

It is better to use GAS modal damping only for M1 and the other mode should be damped at IM stage.

Detail:

I checked the current BS GAS damping control at OBSERVATION state and found that only the 1st modal mode is damped at F0 GAS while the other mode is damped at IMV OSEM.
So, I tested the modal damping performance only with M1 controls.

Figure 1 shows the comparison of the spectra with M1 modal damping (red) and current OBSERVATION state (blue).
Owing to turning of M2 and M3 modal damping, high frequency noise reduces a lot above 1 Hz.
Since M2 and M3 mode is damped at IM stage, we can use this configuration during both lock acquisition and observation.
So, it is better to use only GAS modal damping for M1 and the other mode should be damped at IM stage.

Note:

This configuration would be better for the other Type-B suspensions, so it should be checked later.