I did a comparison between the FARS shield and the Passive Shield and there really isn't much difference between the temperatures. I had replaced my ISS on the VP2 because of a corroded supercap and last summer replaced the supercap, decided I needed to do something with it and added another temp/humid sensor and a standard shield to compare the two. I figured about a 1.3 F difference in temperature between the two.
However, I am collecting data and hope to have a better comparison come next summer.