I don't know what to tell you.. but this is confirmed. I just went back to 1.6.2 and an immediate jump back to the normal hash on nano pool.

https://www.dropbox.com/s/n39hd08zwhwm85k/hash%20drop%20update%201.JPG?dl=0

SRBMiner in 1.6.2 (for this system, 8 cards)
https://www.dropbox.com/s/66l98fk97tnx41l/1.6.2.jpg?dl=0

SRBMiner in 1.6.4 (for this system, 8 cards)
(looks the same, but its just not getting the same share results)
https://www.dropbox.com/s/uky020294zdpbmz/1.6.4.jpg?dl=0