A lot of Linux games are just Windows applications running in a wrapper, which can lead to all kinds of poorly-coded nonsense. But really there's no telling why the GPU usage is so low in so many of those titles . . . at least not without doing some more digging.
To give you an idea how poorly coded some of them are, i have to use a special hack where i compile a shim for divinity original sin to even launch. The developers were too lazy to include proper opengl compliance check, so the game refuses to run on anything, but the old deprecated fglrx driver.... It's a really simple fix and the community have raised the awareness on their forums, but they could not be bothered to spend 10 minutes updating the game so it could actually run on polaris cards by default...
Anyway, i oc'd the machine to 4.4ghz multi and 2ghz NB multi like you said. I also forgot to mention, but the previous results were not completely on stock, as i have lowered the boost clock to max 3.7 ghz, so the results are 3.7ghz boost vs 4.4ghz stable. Here are the 4.4ghz + 2.0ghz results (kernel 4.8.0-30, mesa 13.1):
Age Of Wonders (about a 14 fps increase, probably the most improved of the bunch, but still no where near where it should be)
Divinity Original Sin (still can't force it to show fps, still terrible, laggy performance):
Torchlight 2 (an amazing increase of 5 fps):
Shadowrun Dragonfall (an amazing 27 fps in this scene, hangs on around 30-40 eslewhere):
Trine2 (holds at around 45 fps):
Xcom Enemy Unknown (hangs at around 35 fps):
Metro Last Night (could not force the damn game to go into windowed mode this time, but not much change fps wise):
Day Of Defeat (no change, the game run pretty smoothly, not considering the frame drops of up to a 100, but it still does not go bellow 60. The counter is again wrong, since i exited windowed mode for screenshot):
Talos Principle (not much change):
Borderlands 2 (Not much change):
Unigine Heaven (a bit more stable, but pretty much the same):
Unigine Valley (pretty much the same as above):
Could not test Pillars Of Eternity, as the game refuses to run all of a sudden (it tries to change the resolution and when fails, exits), but it would be the same story i think.
So out of all the games of this bunch, the best performing ones are Day Of Defeat (really big frame drops though), Metro (gets from 40~ to 120~ fps), and Unigine benchmarks. Actually Unigine benchmarks also have the most stable frames (well they drop in the screen transitions, but for a very brief moment).