Freddie H
You have answer it yourself there. 
You don’t need to be rocket scientist to understand if you park something, shutting anything off, it will decrease the performance. Drive V8 engine on only 4 cylinders doesn’t exactly increase its performance. Even a child understands that. 
Freddie you are not even a child then, if an engine manufacturer could make a V8 use 4 cylinders when it's idling they would, it doesn't affect performance when the gas pedal is on the floor and all cylinders are delivering full power when needed.
When having C state, core parking or whatever activated makes very bad impact on any computers performance. As for an example what happens in DAW is that the core that is “park mode”, “C1 sleep” can’t wake up in time when you hit “play” when suddenly the computer needs all its CPU resources. C1 lower the CPU clock meaning slower performance.
One of the side effects that happen if you do have the core park, C1 sleep mode is that you get drop outs and clicks in the audio. Sometimes this can even cause blue screen death in rare cases.
If this is true why am I not getting blue screens pops and clicks when I press play? I can run a 48 samples buffer under a reasonable load without glitching, I choose to run 64 samples and can live with the 7 ms RTL without having to change settings when I change from tracking to mixing. What do I need to fix again?
Microsoft green eco movement politics infinitum madness! 
I like to stop global warming, ecologic when it comes to food, light, lower gas on cars or whatever but not when it’s all about my work computer and performance in the studio. It is better they; Microsoft, Intel, Samsung and rest of companies focus on something’s else;----> lower overall power consumption instead of lower the overall performance of our system and CPU:s.
Choosing the Windows 'High Performance' power plan effectively overides EIST anyway base a plan on that turning off all the going to sleep or hibernate nonsense and you've pretty much done all that's required.
Benchmark your CPU with Core Parking enabled/disabled, C1E (bios) on/off, and then SpeedStep on/off (you'll find that is overridden by the power plan anyway).
Show us the different results in actual figures because here there isn't anything to support turning them off. The only thing that would alarm me is from going from idle to full power if that was manifesting in glitches. It doesn't.
Post #8 in this thread explains it well and highlights the more obvious areas of concern. Some people are cranky enough to hack the registry to get the cores from parking but then install and run the ASUS AISuite, which is a far more evil performance killer than processor core wake times. I hope you're not one of those Freddie!
One final note, just for kicks try benchmarking some stuff with hyperthreading turned off so you are restricted to just the 4 physical cores. The results there might surprise you especiallly on stuff that uses a lot of floating point calculations.
Me, I did all that at build time, now I'd just rather get on and enjoy using my machine.