Trying out XBMC in window mode and noticed cpu was a bit high (about 14% of my dual cores), checking in Ksysguard shows that X is using the cpu. Disabling effects (alt+shift+F12) reduces X's cpu usage to normal (0 -> 1%).

Is this a Kwin/composting bug?

No it's not a compositing bug, it's more a broken concept of X11 and one of the reasons why we need Wayland.

Using OpenGL compositing with "heavy" applications like XBMC adds an extreme overhead. Either use XRender or no compositing at all. Especially in case of an Intel embedded GPU (except Sandybridge and newer) I would not use any of those heavy applications together with compositing.

You can setup a window specific rule to block compositing while XBMC is active.

i may be having a similar issue. with compositing enabled and xbmc fullscreen, the application flickers quite a bit and feels generally sluggish. as soon as i press the backslash "\" key and xbmc has a kde border around it the program seems perfect, no flickers and very responsive. even when the kde window is in full screen. but as soon as i let xbmc take over my entire screen it is slow again.

without desktop effects enabled the program runs very smoothly when completely maximized.

laptop is asus ux31e, intel i5 sandy, intel graphics

Run "kcmshell4 kwincompositing" - in the 3rd tab, is "suspend comspositing for fullscreen windows" checked?
Also disable vertical synchronisation and see what happens.