When building the compiler the same as well as the applications under test with the Phoronix Test Suite, the GCC 4.7.2 and 4.8.0 (a near final source snapshot from a few days prior) were compared on the still-fast Core i7 990X Extreme Edition system with Ubuntu 13.04.
The results in full for this quick Intel Core i7 compiler benchmarking can be found on OpenBenchmarking.org via the 1303230-FO-GCC48INTE76 result file. Within that result file are the 30+ test results along with the full hardware/software system details and logs.
There are some regressions with GCC 4.8.0 on this Intel x86_64 Linux system...
Himeno falls back too with the new release.
Compilation times may be slightly slower with 4.8.0.
C-Ray continues to run faster with GCC 4.8.
View the rest on OpenBenchmarking.org. There's also some ARM performance benchmarks and competing benchmarks against LLVM/Clang for this new open-source compiler update that presents many new features. Tests from other x86/ARM hardware of GCC 4.8 are forthcoming on Phoronix.