Tom, I am not privvy to the details of the development of the JS1 jet but I have heard about some things along the way:

Change to the tolerances of the tiny gap between the rotor and the casing - corrected surging on starting at at typical South African altitudes.

ECU firmware changes.

Increased fuel pressure and smaller injector nozzles - for better fuel misting and starting.

Electronic component change in the ECU - to correct wrong voltage delivery to the glow plug leading to glow plug failures.

The uncertainties that I have relate to the electronics rather than the turbine - that and making sure the fuel is clean.