Yikes. Seems like they are trying to "game" benchmarks.
This results in a game of pattycake where the game developers and the driver developers are both guessing at what the other will do, and trying to optimize for it. So game releases basically require having an NVIDIA/AMD developer sit down, profile your code, and figure out which of the tens-to-hundreds of thousands of behavior triggers and optimizations should be turned on for your game.
What's probably going on here is that CS:GO is having a different set of optimizations applied from its parent game (or hasn't had any at all applied yet on Linux). The big question here is whether CS:GO actually works 100% reliably on those settings, on all AMD GPUs, on all possible combinations of graphics settings. Because it's very possible that it doesn't, or that no one has verified that it does.
[1] http://www.gamedev.net/topic/666419-what-are-your-opinions-o...
A lot of the big games are often optimized on the driver level by both Nvidia and AMD which is why new graphics drivers are released very often compared to other hardware drivers. It's very common to see driver releases the same week as new triple A title that improve performance 40-50% and fix various broken things such as multi-gpu scaling.
In this case it's two games with the same engine (source 1) where the same optimizations happens to work but AMD likely hasn't gotten around to actually verify the optimizations and as such haven't enabled them for CS:GO (Remember steam hardware survey says only ~1% game on Linux so it's hardly worth their time to spend time optimizing Linux gaming)