Hi Peter,
My guess - most people think/believe the engine gauge readings for their aircraft/sports car/truck are accurate - most people would be wrong.
Even expensive laboratory equipment, operating in controlled environments, require routine recalibrations to maintain accuracy, so how can you expect relatively cheap equipment, mass produced, using cheap materials, operating in an uncontrolled ambient temperature/pressure coupled with high vibration & movement, to deliver accuracy.
The best we aviators can expect is a degree of consistency. Even this this is likely to degrade over time.
I keep a log, after each service and then intermittently, of my aircrafts engine instrument read outs, including some flight data (Alt/Speed/OAT) so that any deviations from the "norm" can be monitored or acted on.
I understand the intent of your example "130C indicated is 115 true temp"but as the example could be coolant or oil temperature, I think an indication that is so close/may be in, to the "red" zone would need urgent remedial/mitigating action, no matter what.
