I played a little bit more with the fans today. The CPU fan does run slower when connected on fan header 2. Setting the fan speed in the bios to full speed results 1238 rpm, the silent setting gives 472 rpm. Would be nice if that works on the CPU fan header as well.
I did also try out the stock intel cooler fan that is a 4 pin fan. On full it had 2070 rpm and on silent 1020 rpm. Now I wonder if the cpu fan header just doesn't adjust 3 pin fans or if the other fan just spins to slow for the cpu header pin and the motherboard tries to keep it high.
Now I added the resistor that came with my cooler fan for quieter operation and the fan spins at 921 rpm. Gigabyte's smart fan calibration software again set all workload percentages to that rpm.
BTW, the 3 pinned fan header 4 on the mother board does not regulate fan speeds. That is a known feature of the motherboard.