...Mine are standard roof panels with an aluminum frame... Some air can get underneath, but it's not much. How would that be an efficiency issue?
Rated output is measured at 25C cell temp, which is about 0C ambient. Every degree above that [depresses] the voltage at which the panel makes maximum power (Vmp), reducing output (V*I=W). So if your area is above freezing, surface-mounting will reduce panel output somewhat compared to air-gapped mounting.
The effect is felt most strongly with MPPT controllers, since they traffic in the panel’s voltage range up there where max power is generated. PWM wouldn’t suffer as much because it already ignored those higher voltages.
The amount the panel loses per degree is stated in the temperature coefficient of power that /u/Greeneee- mentioned. You can calculate this heat-related derating. The coefficient is one of the specs listed on the panel’s back label.