Let me share a real world experience with you and you may have a different perspective.

1979 450SL-Failed NoX, you name it was done. Any and all items that you could possibly install to correct the matter simply did little to bring the NoX down. After my friend spent many hours on the vehicle I suggested he take for a nice long drive to see how it would clean up. As luck would have it we have a good selection of decent grades here in the bay area, so off to Santa Cruz. Half way up the hill the engine temp was hovering at 100 C, way too hot for a 65 degree day.

Long story short- the radiator was flowing less than 60%. Installed a new raditator and the NoX fell to the basement.

2100ppm before the rad, <400 after.

Engine temp is something I now consider based on this experience.