I'm running Win10 x64, R9 390 and I've tried the 15.7 WHQL drivers all the way up to the newest 15.10 beta drivers. None of them can stretch the aspect ratio in CS:GO. Some people have reported that using the old 15.6 beta for Win 8.1 works but they all seem to be using 200 series cards or older. Doesn't work with mine and I've read of other people with 300 series cards where the old 15.6 beta doesn't work either. When I tried it I only got a black screen and had to remove it through safe mode.
Every driver switch has been preceded by a DDU clean uninstall to make sure nothing is interfering.
More people with the same issue:
At this point, this should've been heard by the dev team, so I'm not really looking for some miracle fix, more just a confirmation that it's actually being looked at.