I use the same laptop, but I don't use Gnome Shell. From what I remember, Gnome Shell's/Clutter's Sync-to-Vblank option is slowing stuff down with Catalyst (it was an issue on Ubuntu (Compiz), too, which got fixed with Ubuntu 12.04. So try turning that off first.
Also, Catalyst's 2D performance is overall not *that* impressive *yet*, you can try the experimental ShadowPrimary acceleration.
#edit: Just saw that you were talking about the open source radeon driver. Well, that should not happen. 2D is kickass with it for me.