Hi, thanks for your hints. I finally found out, what the problem of the high temperatures was, AppleLPC.kext was not loaded. Adding the device string of the ISA bridge solved that.
My feeling is, speedstep was working from the beginning, just HWMonitor can give very misleading results, SMCMonitor and DPCIManager consistently gave much more sensible data.
However I tried to follow your guide and edited FakeSMC.kext according to http://www.osxlatitude.com/tuning-performance-with-fakesmc-smbios-plistand https://osxlatitude.com/index.php?/topic/7283-apple-mac-smc-keys (I think the x201 most resembles a MacBookPro 6,2). That went fine, but changing from 6,1 to 6,2 in smbios.plist caused a boot failure (errors seemed a bit unrelated, like utun0 error=6).
Since everything seems to work like a charm now, I don't see any benefits from fixing that or should I? Do you recommend also reverting the fakeSMC.kext to the original version?
Since that might be of interest to other x201-owners, can I somehow contribute this "patch" for AppleLPC.kext into EDP?
Greetings, J