it seems that the reason for not reaching PC6+ is iGPU. I've updated my system to kernel from https://cgit.freedesktop.org/drm-intel/log/?h=drm-intel-next to fix screen flickering, and also applied patch from http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=ffb810563c0c049872a504978e06c8892104fb6c (which is a regression fix for kernel 4.6), and now I'm mostly in PC6 and PC8. Also, turbostat showing that GPU frequency is mostly 300MHz, whereas it was more higher on vanilla 4.5.0 and 4.6.0-rc3, so it seems that it was the reason why he whole packages cannot reach states deeper than PC3.

P.S. Applying regression fix only is not enough - it's a drm-intel-next which contains main fix.