So, for all the discussion about DirectX 12, the three main desktop GPU vendors, NVIDIA, AMD, and Intel, want to tell OpenGL developers how to tune their applications. Using OpenGL 4.2 and a few cross-vendor extensions, because OpenGL is all about its extensions, a handful of known tricks can reduce driver overhead up to ten-fold and increase performance up to fifteen-fold. The talk is very graphics developer-centric, but it basically describes a series of tricks known to accomplish feats similar to what Mantle and DirectX 12 suggest.
The 130-slide presentation is broken into a few sections, each GPU vendor getting a decent chunk of time. On occasion, they would mention which implementation fairs better with one function call. The main point that they wanted to drive home (since they clearly repeated the slide three times with three different fonts) is that none of this requires a new API. Everything exists and can be implemented right now. The real trick is to know how to not poke the graphics library in the wrong way.
The page also hosts a keynote from the recent Steam Dev Days.
That said, an advantage that I expect from DirectX 12 and Mantle is reduced driver complexity. Since the processors have settled into standards, I expect that drivers will not need to do as much unless the library demands it for legacy reasons. I am not sure how extending OpenGL will affect that benefit, as opposed to just isolating the legacy and building on a solid foundation, but I wonder if these extensions could be just as easy to maintain and optimize. Maybe it is.
Either way, the performance figures do not lie.