EcuTek does not lock any of the diagnostics. A decent workshop should be able to read codes and values off the ecu no differently to any other WRX.
ECU tuning does not need periodic adjustment like a carburettor/distributor.
If it was once running well and now does not then something has failed or a part that affects the tune has been changed. Vacuum or boost leaks, spark plugs and coils, filters and injectors would be a good place to start. Borrow a good AFM and try it?