After some more research, i can sum up:
If i set the intel_pstate governor to “powersave” in all policies under /sys/devices/system/cpu/cpufreq/policy*/ , the cores will clock down to a clockrate between 800 and 4200 Mhz depending on the actual cpu load.
Oddly, doing so does not have any impact on the power consumption (not even 0.1 watts).
Regardless of the governor, the system still consumes between 3 and 7 watts more than the same system running Windows 10 x64. This sounds like a tiny amount, but is between 20 and 50% of the total power usage.
Changing the power-driver from intel_pstate to acpi_cpufreq by adding “intel_pstate=disabled” in the GRUB boot options does indeed change the power driver, but does not have any impact on power consumption.