The upcoming 'Navi' GPUs from AMD could offer much better power-efficiency according to notes from AMD's recent Linux driver patches. The patches detail that 'future ASICS' could use a new sw SMU framework superceding the current PowerPlay driver. In all likelihood, the new sw SMU might enable dynamic per-core voltage and frequency adjustment found in Ryzen APUs potentially resulting in less power-hungry GPUs and more performance per Watt.
https://www.notebookcheck.net/AMD-Navi-could-offer-signficantly-improved-power-efficiency-thanks-to-a-new-SMU-framework.406176.0.html