Anecdotal, but I've been running Ubuntu 15.10 for a few months on a Dell XPS 15 (9550), and with kernel 4.4 everything works flawlessly.
When idling, power consumption is 8W, and powertop shows 30% C8 and 70% C10.
When idling, power consumption is 8W, and powertop shows 30% C8 and 70% C10.
CPU/GPU power usage: (≈1W) http://geoff.greer.fm/photos/screenshots/Screen%20Shot%20201...
Total power usage: (2.9W) http://geoff.greer.fm/photos/screenshots/Screen%20Shot%20201...
I installed tlp and system went down to 5.5W idle with normal programs open. On fresh boot as low as 4.9W.
Turning the screen off shaves 2.5W, and disabling the wireless card saves ~1W.
I suspect the SSD + HDD are making part of the difference.