> G5 VMX should be compatible to Altivec hence the SIMD unit should get used, too.

Yes, absolutely.

> Mplayer benefits from Altivec, too. But Mplayer gives the way better results on
> the G5. I am not saying G4 Altivec per clock may not be faster, but the main
> difference - so my guess - is the throughput.

Yes, in case of a video player, way higher memory bandwidth likely outweighs SIMD being a little slower. Besides, slower SIMD per clock may still result in faster SIMD if the clock difference is just big enough.