My only proof is that my problems came starting from the new drivers, and all points to X and OpenGL in particular. And seeing that OpenGL is managed by fglrx and according to wikipedia this is a low-level error I make the link.
I can't find more, as I can't look into the black box that's called fglrx.
If something should not be in order, or this is not related to fglrx in a way then you have the freedom to change the bugreport on bugzilla like everybody else with an account.
[edit]
Okay, this is strange:
I ran sax2 without any options and no fancy colours, it just worked!

The "Bus error" is still there and I get KWin's composite effects without enabling the composite extension, AIGLX and DAMAGE. This is really strange.... I'll look at this further tomorrow.
[/edit]