To be honest I am unsure. The Crossfire performance in a title such as Warhead or Metro was good enough to believe that it's working okay. 3DM11 is akin to Unigine in that it's very shader heavy, but that did well too.
So I think it's probably a driver issue, but it's equally possible that only in very extreme loadings (and Unigine is fearsome) does the limitations of the dual-GPU single-card solution become apparent.