I notice in the August 2012 Graphics Card Hierarchy Chart at Tom's, they have the HD 7850 ranked quite high. Higher than I would expect it to be. Are we to assume that ranking is with a PCIe 3.0 board? If so, I would imagine it would be down a bit lower on a PCIe 2.0 board.
I don't imagine PCI-E version would make any difference at all in a card of that class, especially comparing 2.0 and 3.0. I guess they put it there because it is closer to GTX 570/6970 performance than it is to 560Ti/6950 performance. Though in that case, it does raise the question of why the 560Ti 448 isn't further up too.
Oh well, arbitrary distinction for cards that don't quite fit the current tiers set up.
The GTX 680 and 7970 don't even fully saturate all 16 lanes of PCI-E 2.0, so there would be no difference when it comes to the 7850. PCI-E 3.0 might only make a slight difference if you were running the cards in SLI/Crossfire in x8 x8 mode.
The card will perform the same on a 2.0 board as it will on a 3.0 board. And I would say it's where it should be on the chart with the existing tiers. You could make the argument that the 7850 and 560Ti 448 should have their own tier in between the 6950/560Ti tier and the 6970/570 tier. I do see why they probably didn't do that, as that would make a difference of less than 5% between tiers. At that point, it does make more sense to just 'round' the cards in question to the closest performance tier.