One of the reasons cartridge heaters usually have higher watt densities than other heaters is that their installation usually is in such a way that the heat is usually removed quickly from the surface of the heater. If your cartridge heater is extending out of the hole, you only have the surrounding air to remove the heat being generated by the heater. Of course, metal is a much better conductor of heat than air.
This can cause an early death for a cartridge heater. If you have to have a portion of your heater extending past the edge of the mounting hole, make sure you specify an unheated area at the end when ordering.