The point made in the report is that the damage done does not show in the engine parameters or performance. It will only show itself when a boroscope inspection is performed or when bits of the turbine starts flying off.
The report is not an end-all, it only represents the knowledge of the day.
Engine trending over time can show up lots of things according to the model created.
I suspect that standard modeling of EGT and FF would show up the effects of blade erosion and time to refurbish more often.
OTOH I suspect that EGT margins and FF might actually improve if the stuff glassifies in the burner-turbine nozzle vanes this is often followed by engine surge-stalling at an inconvenient time.
Then again the nasty clogging of cooling holes in blades and vanes leads to metal deteoration which goes unseen in trending plots but is detectable by scheduled borescope peeks.
I suppose there is a possibility that if enough cooling holes are blocked that the engine performance (EGT vs FF) may actually improve as the air is deflected away from cooling back into the main stream and thus show up in trending. At any rate I'm sure that the engine trending experts are looking at their computer models specifically for advising operators who actually do this stuff on what to look for.
If anybody comes across some hard facts on this in todays climate please post.