Thermowells have major drawbacks when using "dumb" thermostats. By the time the probe hits the set point, the outside of the beer and the chamber are much colder. I get a much greater overshoot with my thermowell compared to the side of the fermenter. PID based thermostats are a different story.
In my experience you don't get overshoot of beer temperature unless you're chilling 20+ degrees to lager temp all at once. During fermentation beer temperature cycles no more than 1 degree lower. Air is not that conductive, so unless the fermentor is up against the wall of the freezer it's not cooling that quickly.
It doesn't matter what temperature the chamber is; it's the beer temperature that matters. Glycol is cooling the outside of a conical at a much lower temp than the set point.