Regardless of the idle factor, when you pull open descent (or push managed descend when high on profile) you will get the same idle as when you manually chop the thrust levers!
So if that idle figure is safe for minor level changes, why is it not ok (according to the opening post) to close the thrust levers during an overspeed situation?
Again, I would be very careful with reducing thrust in high altitude level flight, but I do not recognize the dramatic spool up figures presented in this thread. Planes would fall out of the sky if things were that bad!