The quick benchmarks coming out over the night are of GCC 4.8.1 stable against the GCC 4.9 compiler that's presently under development. The GCC 4.9 2013-08-18 snapshot was used for testing and built for a release mode from source, just as the same configuration as 4.8.1. During testing the CFLAGS/CXXFLAGS were set to -O3 -march=native.
Among the GCC 4.9 changes to date include Intel Silvermont support, better diagnostics support, continued C++14 support, and much more.
Over the current GCC 4.8 stable series there wasn't much performance change with GCC 4.9 for this Intel Core i7 4900MQ "Haswell" laptop, but the benchmark results and system information in full can be found on OpenBenchmarking.org at 1308220-SO-GCC49INTE96. These Intel Haswell compiler benchmarks can be reproduced by running phoronix-test-suite benchmark 1308220-SO-GCC49INTE96.
GCC 4.9 can produce some performance gains on this Intel Haswell system for at least some BOTAN cryptography workloads.