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!
For Boeing, I can assure you that the spool up time is in the order of 40 secs plus. The EEC changes the spool up time depending on various circumstance:
Analysis of these reports indicates that this is related to the Electronic Engine Control (EEC) software revisions 7.B.U1 and 7.B.V2. Revision 7.B.U1 reduced the engine deceleration and acceleration rates for specific thrust lever transients at high altitude. Revision 7.B.V2 returned the deceleration rate back to normal but retained the reduced acceleration rate. Both versions of the EEC software reduce engine acceleration rate if the throttles are in idle for less than 60 seconds. Slowing the acceleration rate of a heat soaked engine improves engine operability (stall margin) characteristics.
I would advice against using LVL CHG to descent from FL390 to FL370, as you mentioned as an example. Alone from the fact, that it would cause a high rate of descent which may cause nuisance TCAS alerts. If doing so anyway, the ADFS would anticipate the required time to spool up the engines, to level off on speed.