I did not know that they were compatible. Guess I learned my one new thing for today...
Still, a 20% loss is pretty huge. I mean, the loss on a PCIe 2.0 4x slot compared to a 16x is like 10% for SLI/Crossfire, and most people don't consider that worth adding a second card. I probably wouldn't bother with a GPU upgrade until I got a new board. That way you'd be able to take advantage of lower prices and/or higher performing parts.