VLC normally can use AMD graphical acceleration because it doesn’t require a proprietary method - a straight install from either the VLC repo or Packman repo is generally all that is required.
However, I can’t install either VLC or mplayer due to dependency issues (and differing dependency issues in both cases), so I can’t test that out yet.
However, any other applications that normally suffer under lack of acceleration (not just desktop performance, such as kwin, but Flash even more than KDE) now no longer suffer.
In fact, where I typically had to resort to XRender to get decent KDE desktop performance with the proprietary driver, I’m staying with OpenGL and the OSS driver, and performance is better in KDE than it has ever been. Even (perhaps I should say especially) Flash is much better behaved.