After using alamode to calculate the round-trip mode of the beam at the Faraday exit after retro-reflection form the PRM, I'm not able to blame the MMT and TT curvature for the beam ellipticity.
I assume an input waist at the mode cleaner of [0.00159, 0.00151] (in [T, S]). Propagating this through the MMT to PRM, then retro-reflecting back with flat TTs I get
w_t/w_s = 0.9955, e = 0.0045
If I give the TTs a -600 m curvature, I get:
w_t/w_s = 1.0419, e = 0.0402
That's just a 4% ellipticity, which is certainly less than we see. I would have to crank up the TT curvature to -100m or so to see an ellipticity of 20%. We're seeing something that looks bigger than 50% to me.
Below are beam size through MMT + PRM retro-reflection, TT RoC = -600m: