Connected with your diagnosis, using my Dell XPS 15 9550 (32GB, 512 GB nvme SSD) and kernel 4.7.0-generic, if I *use* the Intel GPU and have the nvidia driver installed (361.42) then powertop 2.8 reports PC2/PC3 only. If I *use* the Intel GPU and have nouveau installed (1:1.0.12) then powertop shows PC2-PC8 (45%ish time in PC8). Between the two the baseline power estimate drops from 25W (with nvidia installed) to 16W (with nouveau installed) - and to be clear in both cases I'm not using the nvidia chip, just the Intel GPU.

I have some more notes here: https://ubuntuforums.org/showthread.php?t=2301071&page=32&p=13470668#post13470668

Using 4.4.0 with the nvidia driver (over the last month) I saw PC2-PC8. I didn't understand why this got worse when upgrading to 4.7.0 until a colleague suggested the driver swap (cheers Kyran!).