Hey Opti,
I also like to measure the temperature of the air coming out of the vents but I have an electronic temperature probe to measure it, accurate to 0.1 of a degree for both Celsius and Fahrenheit. Currently after 5 minutes with the A/C being on at the centre vents I get, 6.5 degrees C on level 9 air flow force, 5.5 degrees C on level 5 and 4.5 degrees C on level 1. It gets colder the longer I leave it on but by very small amounts. E..g last summer I left the car idling with it on which doing some engine work for 30 minutes and recorded -0.4 degrees C at the centre vents. I would say yours is doing very well but get an electronic probe to measure it and then we can be geeky together