By setting the lowest C-State form C1 to C2 i managed to save another Watt. (powerd was already running, ofcourse. Still on the default setting though)
On C1 the power usage is 30.3 Watt. With both cores on C2 it dropped to 29.5 Watt. Because 1 core seems to be used, it does not matter if 1 or 2 cores are set to C2. This cpu usage might be because of powerd. It seems to be polling the AC line state every 250[ms]. Also it has to check when the throttle up and down quite often. Maybe this could be changed too.
The harddisks are now to to go to "Minimum power without Standby" after 5 minutes. This saves about 13 Watts in total for 6 disks (43 to 30).
Because I have now added a USB stick for temporary storage (which still has to be synced to less volatile storage with a cron job or something) it should be possible to really spindown the disks. Usually i use my server only in the evening, so spinning up and down once a day should not be a problem. This is normal usage for harddisks in desktop PCs.
There are now 2 active case fans. These are probably not needed.
The harddisks seem to go power saving after like 5 seconds. This is not what i want. With all disks spinning it uses 48 Watts.