I varied the real and fake tag rates for soft muons for the standard model background and dark Higgs signal MC.  In addition, 5% of the taggable MC tracks were thrown away to model the 5% Monte Carlo tracking overefficiency systematic. I then plot the fractional change in background or signal population with respect to the variation for both the W and Z samples. The population plotted is the number of events containing 3(2) or more additional muons in the W(Z) sample.

Note that the signal MC tends to vary in the same direction as the background SM MC.  This implies the limit will be relatively stable with respect to a systematic variation in tag rate.