General Peltier-tivity
For instance, a TEC1-12715 connected to a 12VDC PSU capable of supplying 150 Amps is drawing a mere 60W. Why is this?


Because (if the data sheet can be trusted) at the maximum delta-T, the thing draws 15A at 16-17V.  When you have a passive device, the current drawn is a function of the device's characteristics and the applied voltage.  Usually the current drawn increases with applied voltage.