I have just messed around with an old Lenovo T61 laptop, and tried to get SAC running on the thing. The first thing I did was going into the bios and disabling all the power throttling stuff including SpeedStep, as that had been included in all SAC-tweaking guides I have ever seen - and it makes good sense!
After doing all the other tweaks and installing SAC I had a very stable system, but the CPU usage was quite high even with a moderate number of channels and plugins activated. The pc has a 2.6 GHz Dual Core 2 processor so it's not lightning fast, but I expected a little better performance from it.
As I was trying to tweak things I installed ThrottleStop and to my surprise ThrottleStop showed me that my CPU was running at 1.3 GHz! I then went back to the bios, activated SpeedStep and chose "Maximum performance mode" (in a sub menu to SpeedStep) and now I have the CPU running stable at 2.6 GHz!

So generally SpeedStep should be disabled, but not always!