See https://www.virtualbox.org/ticket/10293#comment:16 . The guest additions build for me (using VirtualBox 4.1.8) in F17 and Rawhide with X.Org 1.12 support (and no need for setting $MAKE to avoid the OpenGL error), so by using hardware passthrough Gnome Shell is much faster. The next release (probably 4.1.9) should include X.Org 1.12 support.
VirtualBox 4.1.10 has just been released, and X.Org 1.12 is supported.