>If the shift is a function of zenith angle, that must mean that the refraction also causes a slight shear, right? Have you estimated the magnitude of that shear? Might we want to include that as well?

Well, the refraction at zenith is 0.0, and the refraction at zenith of 1 degree is a little less than 1 arc second, so that's a shear of few x 10^-4.  The question I haven't settled in my mind is: does this affect stars as well as galaxies?  I.e., does the refractive shear happen before convolution with the PSF or at the same time?