The guys on Jonnyguru would have to disagree. I think if you overclock the 1080s then yes, a 650W PSU is more ideal. But we are talking about 180W GPUs here. Simple mathematics add those up to 360W. A typical CPU, 100W, and that makes your typical system with two GTX 1080s 500W (so that 516W is actually a solid approximation) about under full load.
Good units are designed to be able to run at that type of heavy load. And you'll find in units like the RM550x that the fans blowing as loud as possible is under 40db, making fan noise not even loud. Then you have 750W units that have louder fans running the same hardware. Or some units have better fan curves.
There really isn't any sort of "power overhead" because a power supply won't hit a limit on power (computer ones, that is). You can read about this Jonnyguru discussion I had here
http://www.jonnyguru.com/forums/showthread.php?t=13311
According to the experts on there, a power supply won't ever hit a limit on power unless protection circuitry is overridden, a particular type of load is applied, and a specific internal design is employed that prevents the PSU from burning before the limit is reached. And with the really non-professional Guru3D PSU reviews anyway, I wouldn't listen to their recommendations as is.
I'll agree that 750W is the sweet spot for efficiency with two GTX 1080s. But nonetheless, will run perfectly fine on quality 550W units.