Great postings/questions.
My bit to add would be this;
Topping, (the point at which the ng section is providing the absolute maximum power for the ambient conditions)is your guarantee that the engine will provide all of the flight manuals performance criteria listed.
The various daily health checks provide information on engine trends, not absolute power output. While they usually correlate they don't give the guarantee.
Any factor that might influence maximum power output is being screened on a topping check. Damaged turbine blades, bad rigging, burnt case liner, fuel control scheduling, Np feedback loop etc.
If it tops sooner then the charts say it will then you start the toubleshooting.
Hope to hear more answers too..