How much longer? On a hot day with the a/c running, my temp will rise 10-15 degrees C in the time it takes to sit through one red light following a freeway drive. On a long, uphill grade, is it likely to matter that the car overheats at mile 3.5 versus mile 3.0? Sure, sometimes you might get lucky and catch a little downhill at mile 3.2 and the lower-rated thermostat might kick the can further down the road. But, sooner or later, whatever problem is being masked by the lower-rated thermostat is going to reveal itself.

A lower-rated thermostat is never the proper fix for an overheating problem. Never.