From an older Intel Core i7 990X Extreme Edition "Gulftown" system the LLVM/Clang compiler performance of the 3.2 and 3.3 releases were compared to the current SVN code as of today. There are some LLVM 3.4 performance improvements while for other workloads there isn't much evolution to see.
Anyhow, having out the Intel Core i7 990X system for some updated Linux tests, today I ran some LLVM Clang 3.2, 3.3, and 3.4 SVN 20130715 benchmarks for different C/C++ workloads. The results in full can be found on OpenBenchmarking.org within 1307158-SO-LLVM34COM27.
In one of the SciMark 2 tests there is already a great speed improvement out of LLVM/Clang 3.4.
Most workloads don't see any dramatic shifts yet on the in-development LLVM code, but be sure to see the rest of the results. You can also run your own compiler performance comparison using the Phoronix Test Suite.