Now that I got sick of my laptop being super noisy because of my gpu running at perf lvl 3, I tried the following in my xorg.conf:
Option "RegistryDwords" "PowerMizerLevelAC=0x3"
Option "RegistryDwords" "PowerMizerLevel=0x3"
et voilą - Performance lvl 1 (lowest), even though TwinView is activated and I'm driving two 1680x1050 displays (one via HDMI).
So in the end, this is just another bug in PowerMizer - it just won't reduce clock speed while a secondary display is in use.
This is my last caveat with the nvidia driver, other than that, it pretty much does everything that I want. Except being published under GPL. Ah well