Yes c3/c6 state support is needed for Turbo boost to work fully as it locks the core down to turbo boost the other core.
Does the high pitch sound happen when running off the battery or mains?
It's more hearable when using a battery. On main its not that noisy.
I've asked some people on official forum for my notebook and one of the
guys told me that the c3/c6 support is only needed to some power savings
I really don't know what to think about it now
Yes it is a power saving feature but its also the way Intel tied their Turbo boost to work fully by locking a core down to increase another cores clock speed in order to not increase the power needed.
With it disabled Turbo boost will only boost all so for a i5 460M @ 2.53 GHz for all cores go up to 2.67 GHz but it will not go upto 2.8 GHz for one core unless you enable c3/c6 state support.
Thank you very much! You fully solved all of my doubts regarding c3/c6 and its influence on Turbo Boost
P.S. Now I know not to believe in all what they say on official forums.