Having Speedstem and C-State enabled means that the processor is able to use power saving features and the operating system is able to inform the processor of work and detect the current power state of the processor.
The power saving features available are things such as downclocking the processor so a 2.4GHz processor could, when idle, be running at 800MHz, it could reduce the voltage required for components as they are in a lower power state and other features.
Coming out of these low power states could, in theory, make the system feel sluggish for a brief moment but in my experience it is not even vaguely noticable (I have an I7-860), my system cleanly migrates from low-power to high-power states with no noticable lag or delay at all. My system is fully responsive and shows no signs of choppiness or being "slow" even when operating at the lowest frequency, which it does most of the time when I'm just browsing websites or similar.
One other feature of the C-states is that they (I believe) are a requirement for the proper functioning of the Turbo-boost feature (I think I read it on a datasheet or something) which can actually improve processor performance by a good amount on single-threaded tasks as it allows unused cores to be turned off in order to allow a working core to self-overclock and run beyond it's rated speed for brief periods.
In the end though, enabling those features should save you power.