ffmpeg and associated projects do not rely on autovectorization, which pretty much only works on naive scientific code. They write their SIMD in assembly and don't need to care about compiler settings.
If you're compiling hand tuned assembly, -march and -mtune probably will have more effect when compared to compiling and optimizing C/C++ code.
OTOH, I'd like to underline that heavily optimized scientific code and libraries are neither naive (in terms of algorithmic complexity/implementation) nor straightforward :D
