Yeah, I had an issue with massively slow hashrate on a brand new 390x. Had the newest Crimson installed, bumped back to 15.something and my hashrate went to where it should be. By bad hashrate I mean it was only putting out maybe 10 and it was never recognized by the pools. So what I am saying is you need to do a search for a version of Crimson that will function properly for your setup.
For me, the Crimoson cannot regcognize 8 GPU in my 4x7990 set up. I used the lbr 7+ driver mod, did not work.
For my 4x7990 setup, with Catalyst 15.11 or early, the 8GPU were recognised instantly, there is no need to mod.