Since clang 3.5 (or possibly even earlier), clang has basically been neck-in-neck with GCC. Most benchmarks seem to show the two basically equal to each other, and then there's a handful where clang wins outright and a handful where GCC wins outright. [1]
For my own use-case (a high-performance photorealistic renderer threaded using TBB), my clang builds outperform my gcc builds, but of course that's completely anecdotal and based on just my own use-case.