I rooted the installation and installed AntuTu CPU Master. This shows indeed that the kernel scales the CPU interactively from 400 to 1400 MHZ, so my assumption was wrong. I now forced it to 400MHZ, effectively not doing any interactive CPU scaling at all. I will see how this works out with regards to power drain.
With moderate/low usage it can last 3 or 4 days (flight mode at night). If you have bad reception (weak signal) it can go down to 24 hours. Check your signal. Here it happens that it jumps between 3 station and always sends with full power - this drains battery.
You can also give BetterBatterStat (BBS) a chance to find out which app might prevent the device from sleeping (wakelocks).
Cheers and thanks to all the devs. The only I miss is the compass which loses calibration ....