Hi all,
thank you again for the suggestions!
First thing: VGABIOS. I was able to extract the blob (the hint in the wiki is a bit hidden and the introduction of the section is a bit misleading btw) and re-ran the measurements using the proprietary VGA BIOS:
Idle: 13,4W Stress: 37,2W
So they values are exactly the same as with native graphics init.
Second thing: Powertop. I did the suggested powertop --autotune experiments in idle and obtained the following results:
vendor BIOS: 8,8W Coreboot: 12,2W
So in both cases a decrease of around 1,2W.
But still something else drains these extra 3,5W. Not running in C3/C4 would explain that difference maybe. How can I check that? I am now doing another attempt to include the microcode updates, but I doubt this will make such a big difference.
Cheers, Daniel