Boiling point is raised by the pressure so whatever you've heard before is not correct. That is why systems are engineered scientifically. Water under pressure takes more heat. The problem is gearing, tire size, tire width, tire weight, vehicle weight, oil, percentage of road grade inclined or declined ect... ect... The added heat of the resistance caused by all these variables changes the engineering design and calculations minimally can be drastic. The system is designed to allow water at the correct percentage of coolant to work properly. The coolant must stay in the radiator long enough to allow the coolant to exchange the heat carried. When the temp or load increases the cooling period must also but in fact decreases because the thermostat is now open allowing the water to flow freely and keeping it from sitting in the radiator to cool. When this happens the heat in the water (temp) increases exponentially making it almost impossible to cool. Cures are bigger radiators if room allows, including slower water flow (not a good choice) hence the high thermostat temp ratings for XJ's, or removing some of the added variables. I have seen secondary radiators once on an FJ40 Landcruiser roof but what a pain to burp and run efficiently.