A week or so ago when I was fiddling with collatz on a 270x I went and updated the driver to "AMD Vulkan Radeon Software Beta Feb25" which windows says is 15.301.1901.0 dated 2/26/2016
This driver seemed to be a great update for the 270x. COllatz CPu usage was way down and as far as I could tell the run times were the same. Yesterday I updated a 7970 to the same thing, it's running off site so I can't tinker at the moment. After checking on its milkyway results I noticed run times are nearly double, if not more. Unless I can figure out something else causing this problem I'll be going back to whatever the previous driver was the next time I've over there.
Keep an eye out if you do update your amd drivers, and if you do update and have some observations please share em in here.