I see that you are using -O3. Clang has -O4, which uses -O3 and LTO. Is it possible to use this flag?
I would like to see a benchmark with more "distribution like" compiler flags like:
-march=x86-64 -mtune=generic -O2 -pipe -fstack-protector --param=ssp-buffer-size=4
It would be nice to see, what are the differences for distributions between Clang and GCC. I think most people are using software compiled with O2 and march=x86-64.