I have four Weber factory thermometers on various grills. Three of them were spot on when tested and one measured almost 20 degree too low. It is really hit or miss with any of them. The only way to know is to test them and even then you are only normally confirming them against one particular temperature. I personally don't worry too much about it. For high temp grilling I only use them as an approximate measure and to track if my fire is maintaining temp. For BBQ, I am normally using a digital along with the mounted thermometer. In that case, it is only a backup as I depend upon the digital to track my actual temps.
WSM, 99 & 09 Performers, 2-OTG, SJG mini WSM, Summit 420, Q100, Akorn, and Schwenkgrill