Any thermostat will open at the given set opening temp. But they are not fully open until they hit 15 degF above set opening temp. So a 160 will open at 160 but not be full open til 175.
For every pound pressure cap a radiator has , it will boost the boiling point of water by 3 deg per pound. So a 15 lb cap will boost the boiling point by 45 deg added to the boiling point of water at 212 and you have 257degF before boiling.
A zero pressure system will alow water to evaporate faster because there is no pressure.
Sending units for most gauges is located at the front of the manifold near the top hose outlet. The sending unit is picking up the coolant temps as it passes that sending unit. The coolant is also at its hottest as it passes ,but it is also coolant that is leaving the motor, not whats in the block. If the pump, fan, radiator are doing there job, coolant temps in the motor should be cooler than what the gauge is reading.
Just passing on some cooling knowledge....