I compiled libcapnp with the latest Clang g++ that ships with XCode. It perhaps would be more fair to also compile the C++ benchmarks with Clang, instead of the Macports gcc4.8 that I'm using, but unfortunately Clang barfs on some template hackery in the benchmark driver.
That would be quite useful. Otherwise it's very hard to tease apart the differences between optimization back ends of GCC and LLVM vs the Rust and C++ front ends. I feel like I have seen benchmarks showing differences around 5-20% between LLVM and GCC in speed, so the it will have an effect.