But what makes you think that downturn RPM-increase is increasing the temperature on the glow-plug ?

The temperature will increase if you pump more fuel proportionally to the RPM increase and that is what
happen if you are dependent on the pressure in the exhaust pipe.

On contrary, if you are not dependent on that pressure, the glow plug temperature should go even lower
beside the RPM increase, because of the increased amount of air in the mixture. I.e. that would lean the
engine without (!) overheating it.

In other words that pipe pressure makes sure that the engine does not go lean in downtuns and burns
more fuel so you have to throtle back, which most of the pilots do.

Thus, the Microcontroller inside of the onboard driver might do better job and you do not have to throtle
back; the RPMs will go high but the engine will stay as cool as on normal RPM even more cool maybe.
This should be measured indeed.