It's T460s with i7-6600U (exact model 20F9003WRT, CPU microcode version 0x84). I could reach PC7/8 every time I'm on battery, on commit ba3150ac3876acd082307f142597d3482107facc of drm-intel + commit which fixes 4.6 regression. I'm also using TLP which forces ALPM (min_power) and setting energy_perf_policy to powersave. Hope this will help you.
Re: The reason is iGPU