Phoronix did some testing with the free driver and AMD's closed source. Seems like AMD only optimize for unigine...
CS:GO runs fine on the open driver.
At 1080p, the Counter-Strike: Global Offensive performance is very close between these two AMD OpenGL drivers, but at 4K, the open-source RadeonSI Gallium3D driver is doing much better. It will be interesting to see if this changes with the summer roll-out of CS:GO on Source 2.
As Wendell says, this is with Linux kernel 4.12 and Mesa 17.2-dev that is fresh straight from the Git repo. Doesn't get any more bleeding edge than that.