I'm not a hardcore gamer, so this kind of things are not my main strength, but the OpenGL argument makes sense. You should try the game on Windows with OpenGL to make sure if that is the cause.
my experience is that the frame rate and consisentcy of frame rate of MOST BUT NOT ALL games through Steam, Desura and standalone has varied and I dont mean per title aI literally mean per second (so 5,12,60,80,12,25,35,36,36,48, etc..) I have messed with Vsync on KDE , AMD's tear free options, AMD's Vsync options, Game Vsync options etc..
Now some of this is down to poor ports of games with bad opengl implementation (killing floor) but mostly it would seem there is some GPU clocking issue or Refresh issue as the frame rates can be very high at times ?
Again, I'm no hardcore gamer, it's pretty much out of my expertise area. However, might have something to do with game specific optimizations on the drivers? I'd look for a changelog. If that's the cause, it's likely to improve with time, now that this games are becoming more usual on Linux.
a quick example = CS:source which can run at 120+ FPS on windows yet runs about 70-75fps on linux? all closed source drivers. So most people wont notice because their refresh is 60hz on the monitor. TF2 is only around 30fps-40fps but again this is upwards of 80fps on windows. Same drivers as windows as on linux (in version at least)
The open source drivers are not likely to have better performance than the closed source ones, it's usually the other way around (there are huge improvements on the releases you name, anyway).
So i would say no, the performance is knowhere near to windows with closed source. Of course im excited for Mesa 9.2 and Kernal 3.11 to merge and then i shall try the opensource drivers..
which finally leads to my point.. opensource drivers are fantastic on the desktop! they beat out the closed source for stability and smoothness.