I have not seen anything official about it, and i have searched.
At the end of the day however there is literally zero difference in PCIE 2 vs 3, as a single cards do not saturate the PCIE bandwidth as it is. I remotely recall seeing something where someone had like a 2% difference, if I recall right it was Crossfired 7970s pushing 3 displays at high resolution and max quality settings.
Two percent can be made up by adjusting PCIE bus speeds and HT speeds so really, it's kind of moot, at least in the current generation of cards/games.