I used my simulator to generate nine realizations of the same field, each with eight observations with conditions drawn from the OpSim database. Each of the nine sets of simulations had the same range of airmasses and parallactic angles, but different ranges of seeing. The first set was forced to have constant seeing by forcing the seeing for all eight observations to the minimum from the database. For each successive set of simulations, the seeing values were scaled so that the maximum was 10% greater than the previous simulation.
I ran two reruns for each set of simulations, one using CompareWarpAssembleCoadd and the other using DcrAssembleCoaddTask. I ran source detection for all 18 reruns, but copied the source detection catalog from the constant seeing DCR coadd run to replace the detections for all the other runs. Thus the source measurements were performed at exactly the same locations for all runs, and in the same order.
To compare different runs, I first removed any sources that were flagged in any of them. Then I normalized the aperture fluxes by dividing by the PSF flux from the constant seeing run, and took the median from all of the unflagged sources.
In the figure below I plot that normalized aperture flux as a function of aperture size for several of the simulations with different seeing ranges, and for both the CompareWarp and DCR coadd algorithm. In the case of constant seeing (the darkest solid and dashed lines) the aperture flux is identical for the two algorithms. As soon as the seeing is allowed to vary even 20% the CompareWarp algorithm (solid lines) underestimates the flux by 5%, though the value is consistent even up to the largest range in seeing. For the DCR coadd algorithm (dashed lines), the fraction of the recovered flux within an aperture degrades much slower at first, but does so steadily.
A key takeaway is that the 9" aperture flux is almost unchanged for DCR coadds as long as the range of seeing is within 50% of the best seeing observation.
The above plot uses the PSF flux from the constant seeing run to normalize the aperture fluxes of all the other runs. The PSF flux also changes for every run, so an alternate metric would normalize the aperture fluxes by the PSF flux measured on the same coadd. This effectively corrects for the loss in power seen in the CompareWarp coadds, and I have included the combined plot below for completeness.