KWin in KDE SC 4.6 is not going to change anything radical, but there are some scripting interface changes and some performance improvements within the compositing engine. With KWin 4.6, texture and geometry data is now always cache and the GLX_EXT_texture_from_pixmap extension is utilized rather than QPixmap/Qimage.
According to Martin, there are great improvements for effects that transform a single window, since these KWin effects no longer cause the entire window to be repainted. There's also an optimization for the Lanczos filter and other improvements.
Martin talks about KWin 4.6 in this blog post.