As some quick benchmarks of GCC 4.9.0 stable versus the latest GCC 4.10 snapshot, I ran some C/C++ compiler benchmarks under both compilers from an Ubuntu 14.04 x86_64 system running an Intel Core i7 4790K "Devil's Canyon" high performance processor. Both compilers were configured and built the same for the system and the same CFLAGS/CXXFLAGS were used during testing.
Long story short, there isn't too much to see from the GCC 4.10 performance at this time over GCC 4.9 for the selection of benchmarks used on the Intel Haswell processor.
In only a few benchmarks was there any change in performance while for most of the tests the results were flat. You can see all of this very early GCC 4.10 compiler data via OpenBenchmarking.org with 1406147-KH-GCC410TES62.