Last I checked (which was a while ago to be fair), LLVM machine code quality still lagged behind GCC - so things should be slightly more interesting with the GCC back end.

There were also some bugs (hence disabled optimization passes) and missed opportunities from the lack of aliasing Rust precipitates - again, not sure where those sit - and GCC will have to play catch up here (unless there are other languages that exercise this part of the backend).