NOTE: There was a small bug in my initial calculation. The plots and numbers have been updated with the fixed values. The conclusion remains the same.
Using Nic's a la mode mode matching program, I've calculated the PRC mode and g-parameter for various PR2/3 scenarios. I then looked at the overlap of the resultant PRC eigenmodes with the ARM eigenmode. Here are the results:
NOTE: each optical element below (PR2, ITM, etc.) is represented by a compound M matrix. The z axis in these plots is actually just the free space propagation between the elements, not the overall optical path length.
This is the ARM mode I used for all comparisons:
This is the nominal "as designed" PRC, with flat PR2/3 folding mirrors.
This assumes both PR2 and PR3 have a RoC of -600 when not flipped, and includes the affect of propagation through the substrates.
In this case we only flip PR2 and leave PR3 with it's bad -600 RoC:
I left out the current situation (PR2/3 with -600 RoC) and the case where only PR3 is flipped, since those are both unstable according to a la mode.