When my 1985 M-B had an overheating problem, I tried using an IR gun, but never got consistent readings which helped diagnose. But, when I shot at a T-stat in a pot of hot water, it gave amazingly close readings to a thermometer in the water, even shooting thru the water. Keep in mind that its field of view used to average the IR emitted light to determine temperature is larger than the small laser spot, so you must insure the surface you are shooting covers its view (hopefully manual documents). Also, the reading can depend on the emissivity of the surface, more accurate for black surfaces, though mine (cheap HF) seems to read fine on aluminum surfaces (uses 2-color method?).
Unless the radiator is clogged with hay, or a shroud restricts the flow, airflow when driving on the highway should suffice, so your clutch-fan likely isn't the cause. More likely, something is limiting water flow rate. Many hobbyists claim that restricting the flow can increase cooling. I'll just note that the T-stat works the other way, and that idea has no basis in heat transfer theory.
The problem in my M-B turned out a clogged radiator. Before getting a new radiator, I had tried everything over a year (T-stats, water pumps, citric acid flushing, used flushed radiator, clutch-fans, ...). The problem was that the temperature would creep above setpoint when idling a long time at a stoplight, then drop when driving. Normally, that fingers the fan, so seemed strange. I popped open the OE radiator and found just a thin metallic-mud film covering many of the down-tubes, and it flaked off easily. Strange it hadn't when I ran a grill brush over the top from outside.
