See the discussion of pros and cons, below. See also discussions of the seismic noise, internal thermal noise, and radiation pressure, below. 500 Hz curve in the radiation pressure noise nominated region below that. This is because the radiation pressure noise is due to the carrier power in the arms, not the signal power out the dark port. During upgrade installation, these numbers will, of course, all be remeasured carefully.
White space means unstable FP resonator cavities.
The beam waist is in the middle of the 38.25 m arms. I don't know how to avoid this, or what its consequences are. I hope we can use the existing 1 meter fused silica spacer currently in use at the 40m, as well as the existing spring mounts. Here is an optical design for a stable, symmetric cavity that provides good rejection of higher order modes. I do not know how to set, or address, the specs on pointing accuracy, jitter, etc.