A Study of the Polarization Test Image
'science' investigation of SG's simulations of the Crutcher test image
Fetched gunzipped and untarred the images.
They have names like
i19-aca.fits
i-aca.fits
i-alma.fits
i19-alma.fits
q19-aca.fits
q-aca.fits
q-alma.fits
q19-alma.fits
u19-aca.fits
u-aca.fits
u-alma.fits
u19-alma.fits
I will assume that these are I,Q,U images of the simulated test image with pointing and phase errors superposed.  I will assume that the '19' refers to a 19-field mosaic whereas I will assume that those not so labeled are a 7-field mosaic.  I will assume that those labeled aca were simulated assuming ALMA + ACA + TP12m and those labeled alma were simulated assuming ALMA + TP12m.
To determine the fidelity index of these images I could do the simple task
                    Model
                         Fidelity = ----------------
                    Max(difference, 0.7 x rms(difference))
and      
                         Max(abs(model))
          Fidelity range = ------------------------------
                                                             Rms(difference)
but I don't have the model.  So perhaps a deviant index might be defined:
        Deviant index = ALMA - ACA
                                        ------------------
                                    ALMA
For the intensity images, first that with 7? Fields:
The difference image ranges from -.6 to .7 whereas the input ALMA image ranged from -.5 to 8.7, so the differences are less than ten per cent here.
The deviant index shows some deviation near the edges of the image but in the central regions tends to be somewhat negative on the W and somewhat positive on the E, with values like
1.7 to the SE near pixel (227,223) and -.06 to the W near pixels (275,257), in an image clipped at the roughly 1 sigma level.  The peak is near where the peak of the difference image lies, of course.  
For the Q image, also 7? Fields.  The difference image shows stripes oriented at pa about 40 degrees with a bright band in the nw and a dark band in the se, with two bright and two dark bands overall.  The difference image ranges between -.077 and .043, following the stripes, while the Q images themselves ranged between -.13 and .055 for ALMA and -.10 and .065 for ACA.
The difference image has its highest positivity where the Q image is most negative.  The deviant index image ranges between -1 and 1; much of the image is blanked since I used a 1 sigma mask in forming the ratio.  The ACA image differs quite visibly from the ALMA+SD image, being fairly smooth with lower values toward the SE and higher values toward the NW while the latter has a sort of yin/yang character with more variation in the center of the image.
For the U image, also 7? Fields.  The difference image shows a dark doughnut structure, with a strong central peak surrounded by a dark halo.  The range is -.04 to .06 while for the U images themselves the range lay between -.03 and .2 for the ACA and for ALMA both.  The image appearance of the U images is clearly different to the eye, with the ACA structure possessing a broad centrally condensed aspect while the ALMA structure seems more clearly divided into three regions to the E, W and S of center.
In my opinion, the Q and U images show greater variation between the ACA and ALMA cases than other images I have seen in these simulations.
Dick Crutcher sent .ps copies of polarization images for the four cases.  Comparing ACA and ALMA, it is very hard to see a difference qualitatively, by eyeball.