Compiling your kernel of numerical code fast is one thing, and there are certainly compilers that specialize in that and all kinds of cpu architecture extensions to take advantage of it.
Where I have seen gcc excel is in making the work-a-day integer code lean and tight and cache friendly.
Most codes are integer codes and we need good work-a-day compilers not the hotrod numerical code wonders that are fast when the compiler does not ICE or generate incorrect code.
With luck clang can catch up to gcc and be a good work-a-day compiler but it seems clear that clang is not there yet.