After reading your post a second time; 7 - 8 minutes is not enough time for the burner to get to maximum heat. Do the test again, but wait at least 15 minutes for the burner to heat up.
Though it is a Dot, a penetration probe thermometer is not as accurate as a infra red for measuring surface temperatures. I believe that information is somewhere on the ThermoWorks site. They even mention that it is not the best thermometer for air temperature. For that you should get the high temperature air probe. How much of a discrepancy between using the Dot with a penetration probe verses an infra red I couldn't tell you. I may test that today using my Chef's Alarm and an infra red.
If the thermometer is accurate, a common reason that the burner is not reaching at least 550°F was covered in Manxman's post. Such as have you added any appliances on that same circuit, that may be drawing more current? Also the build up of residue. But it may be as simple of too much moisture in the bisquettes. I realize you said you are using bisquettes that you had in storage for a long time, but another cause may be how the bisquette was manufactured, and/or the type of wood. There was a time (I don't know if they still do it from time to time) they increased the amount of compression during manufacturing of the bisquette, to slow the burn rate. These bisquettes would only burn halfway through. As to the different types of wood, I find that my apple, and maple bisquettes seem to burn much faster than oak, and hickory.
