Dell Precision 5510 ( pretty much the same as the XPS15)
i7-6820HQ,
Samsung M.2 PM971 (NVMe)
Fedora 23, Rawhide kernel - 4.6.0-0.rc6.git1.1.fc25.x86_64

I was only getting to c3 ( package ), but managed to get to C8 by disabling the Nvidia Quadro, once I found out where to get bbswitch-dkms from

https://fedoraproject.org/wiki/Bumblebee

For anyone wanting to repeat ..

dnf -y --nogpgcheck install http://install.linux.ncsu.edu/pub/yum/itecs/public/bumblebee/fedora23/noarch/bumblebee-release-1.2-1.noarch.rpm
dnf install bbswitch-dkms
modprobe bbswitch
echo OFF > /proc/acpi/bbswitch

Package
C2 (pc2) 4.8%
C3 (pc3) 2.6%
C6 (pc6) 3.9%
C7 (pc7) 0.0%
C8 (pc8) 18.4%
C9 (pc9) 0.0%
C10 (pc10) 0.0