I went from GTX 680 (official Mac version) to GTX 1080 FE and noticed reduced power consumption under load and lower internal system temperatures. Switching from single CPU to dual CPU for daily usage recently had minimal impact on power consumption. Removing the last spinning HDD from the system several months ago also helped lower internal temperatures.
At startup, machine is 200-225 watts for 30-45 seconds, then kicks up to 300-475 watts for 10-20 seconds, then 275-350 watts for 20-30 seconds and hovers around there most of the time. It will get as low as 250 watts if not doing much with the machine for extended periods of time.
When just using the system for "normal" work without video rendering or heavy GPU taxing work, it's 250-325 watts on average. When taxing the GPU (and/or CPU), system will usually spike around 525 watts. I'm not watching the meter all day every day, but I suspect it does not ever get much higher than that.
System config at the moment: Dual 3.46 CPUs, 128GB RAM, GTX 1080 FE (with dual mini 6-pin to standard 8-pin power connector), all SATA bays filled with SSDs, Apricorn Velocity Solo X2, Apricorn Velocity Duo X2, USB 3 card.