Okay. I was just aware of the generic "processor.max_cstate=2" parameter. I tried with both parameters using the vendor BIOS and here are the results:
Can you do a test run with closed devices? This would ensure, you're not measurring a brightness difference.
Okay, you mean with the display turned off? I did this now using xrandr and here are the results:
Coreboot: 9.0W vendor BIOS: 6.9W
Cheers, Daniel