Clipping works now using XRender, although I am still confused why the original X11 pipeline did manual clipping in its blit routines. It still seems to work as-is without it :)
For now only cases work, which have been previously accalerated already by the existing X11 pipeline, however now also with the XRender stuff.
Yesterday I tried Fedora9 to see how far EXA (more or less a requirement to have fast XRender) has advanced, and I was surprised to see how well it was working. However Fedora9 itself ... will have to way until the first batch up updates are released.