Check your IR gun against a known thermometer at the 38 deg temps. My Fluke IR gun is off about 10 degrees at those temps, but right on at 75 deg F.
And unless the return air was 80 or more, I would suspect the 104 deg reading as well. Heat pumps rarely have more than a 20 deg temperature rise particularly at low outside air temps.
And finally, not to be a smart&*%, it is not the efficiency that drops off. My home heat pump works down to about 15 degrees outside air temperature. But the seawater will freeze in the condenser of a marine unit at much below 40 degrees. That is why Dometic and other marine heat pumps have a sensor that cuts off at that temp.
David