FWIW ..
I think the phoronix benchmarks got similar results.
http://www.phoronix.com/scan.php?page=article&item=gcc_llvm_clang&num=1
http://www.phoronix.com/scan.php?page=article&item=llvm_gcc_atom&num=1
IMHO, 10% is not a huge performance difference though.
Pedro.