If you compile and am not seeing any difference, got to the KDE desktop, bring up a terminal and try to play mplayer using the following:
mplayer -vo vdpau -vc ffh264vdpau <file> or
mplayer -vo vdpau -vc ffmpeg12vdpau,ffh264vdpau,ffwmv3vdpau,ffvc1vdpau <file>
While compiling did it replace the original mplayer binary?
If no, then you would have to replace the mplayer binary with the new one.