Not too sure about your car specifically, but generally Mitsubishi have a 2-wire temp sensor for the ecu, and the sensor for the meter is actualy a seperate 1-wire sensor, and the gauge is calibrated in such a way that it will show 'normal' over a wide range of temperatures, and will only shoot to H suddenly when it passes a certain temperature, and because the sensor is only 1 wire, the accuracy of the gauge also depends on the chassis and engine grounding. So basically, the stock temp gauge is generally a decent enough indication for overheat, but if you want to be really sure, it is best to install a seperate temperature gauge. Also usually the stock temp sensor detects the water temp at post radiator entering the engine, and not the temp of the water exiting the engine entering the radiator. If you install your own gauge, it will be on the top hose which means you will be reading the higher temperatures of the water exiting the engine entering the radiator.
But then again, looks like you had found your culprit, so just change the radiator and see how la, but if you still experience the same issue again, check your obd water temp see if it tallies up with the gauge reading.