I've been rolling back between the Omega 14.12's and 15.5 betas a few times on my R9 280x. I've finally decided to stick with the 14.12's as they seem to have a more consistent framerate.
What i've found with the 15.5's is that sometimes, you might be looking at something in the distance or just in a general direction and the frame rate will just drop 20% or so.
The 15.5's are slightly faster for me, but they seem to have unexptected frame drops (around 20%) which I usually fix by just looking in a different direction. Two cases I can recall:
Crookback Bog ~38fps -> 31fps (look around a bit and back to 38fps it goes)
North of Novigrad (Gustfields?) ~45fps -> 35fps (looking around doesn't seem to fix this, but bizzarely if I extend the tree render range the framerate goes back up to 45fps (FoliageDistanceScale=1 -> FoliageDistanceScale=1.2). Installing the Omega 14.12's keeps the framerate at a constant 45fps for the exact same location with the standard foilage distance.
Rolling back to the Omega drivers drops my maximum and even average framerate slightly, but I don't have those performance drops at times (I'm rather pedantic and have RadeonPro's FPS counter on 99% of the time I play the witcher 3. My favourite mini-game is to keep looking at the FPS counter when things start slowing down :)
Of course this could be due to any number of configuration issues on my machine, but it is pretty consistent switching between 14.12 and 15.5 beta (I've reinstalled the 15.5 driver about 3 times before deciding that 14.12 was the way for me to go).