While the Ubuntu Mainline Kernel PPA ships with the AMDGPU DRM driver enabled, it doesn't yet enable the new PowerPlay Kconfig option for getting faster performance on hardware like Fiji, Tonga, etc. Their kernel also doesn't ship with the experimental CIK GPU support enabled. Thus I spun a 4.5-rc2 kernel this morning that enables these extra AMDGPU tunables.
If you want to use the AMDGPU driver with a Sea Islands / CIK GPU (GCN 1.1 like Hawaii, Bonaire, etc), you still need to blacklist the Radeon DRM kernel driver. Additionally, you need to ensure you have downloaded the very latest Radeon firmware microcode blobs otherwise the AMDGPU driver can't initialize the hardware.
With regard to the PowerPlay option, while this kernel spin enables the Kconfig option, you still need to boot your kernel with amdgpu.powerplay=1 to actually enable this power management support at runtime.
If you want to give this Linux 4.5-rc2 kernel a try on Ubuntu x86_64 boxes, you can download it at Phoronix.net. Shortly I'll share my R9 Fury test results to see if the performance improved at all for me on this weekly kernel test release.