On the other hand, sometime in higher level languages the compiler could prove more constrains and then apply more aggressive optimizations. (I don't remember a specific case now, perhaps we still need a few years for an interesting example.)

Another possibility is that using a higher level language it is possible to write more complicated code, with smarter optimizations, because the programmer doesn't have to take care of all the details. For example, the Pypy project they has a Phyton interpreter written in a combination of RPhyton an C that is faster than the standard Phyton interpreter that is written directly in C.