Oke, so when I enabled the speedshift comp froze and crashed immediately (tried twice). So I undervolted -0.100v, like you said, (might go to -0.125v or so later) and get the green SST without a problem now. And I changed the System Agent Voltage to be like the iGPU.
I changed the batteryprofile to also undervolt until -0.100v and it doesn't crash anymore when I pull the powerplug.
About the C0%, it's usually around 3%, from 1.7 to 5%. I'll take a look later what I can turn off.
So I turned off afterburner and some anti-adware. The C0% is now around 2.5%, however I wouldn't know what more to do. The rest is anti-virus and drivers for mouse, keyboard, etc. I don't know about Dell, I have an MSI ge72mvr.