There has to be a big reason why the open source driver is doing so poorly compared to AMD's. PCI-E 2.0 will give you 1-3 fps increase. I remember when ATI made the Radeon 8500, that John Carmack found a bug on their driver that made a huge difference in performance. Can't remember what the bug was.
Once the OpenCL code is running full speed it will become more important to get that supported, but it's not an urgent need yet. Better to focus on other performance optimizations and power management for now.
Guys, I hate to say it, but PCI Express 3.0 is out...
And it offers no gain in performance on even the fastest 16x GPUs on the market, you'll only see a difference on marginal 1x and 4x boards.
You can prove this your self by taping over the pins on the card to step down to 8x, 4x and even down to 1x and see how little impact it really has. No, it wont harm the card, the extra lanes don't provide the card with power, only data, all slots be they 1x or 16x supply the same amount of power.
only way you'll see a difference is via 3x or 4x crossfire via the PCIe lanes. But there is no Crossfire support yet, is there?