Driving TEC below rated voltage
TEC performance is pretty good in low voltages due to lower ohmic losses. As rough approximation, TEC's heat transfer is proportional to its current, but ohmic self-heating is proportional to square of current, so at lower voltages TEC produces less own heat per unit of treansferred heat, so it is more effective at lower voltage.

Thus, it is better to drive TEC with variable voltage/current to control its power than use PWM.

But TECs have relatively high thermoconductivity, so in some circumstances smaller (and less powerful) TEC is preferred in order to reduce thermoconductivity losses.