I always thought that opensource ati drivers are not perfect for running gnome-shell, but I was wrong after a clean installation of Arch Linux.
Since Arch Linux is a rolling-release distribution, all of its packages are always up to date with the latest stable, and maybe that explains why. So I am running the latest xf86-video-ati drivers and everything in gnome-shell is flawless. The transitions are smooth and there are no hangups. Everything is perfect!
I love gnome-shell and xf86-video-ati!