There are some significant issues with both Pango and Cairo over the transition from ATSUI to CoreText, particularly with Pango.
We (meaning Paul Davis of Ardour and I along with a few less-consistent kvetchers) have badgered one of the Gtk+ core devs,
Kristian Reitveld, into doing some work on the quartz implementations and getting patches incorporated. So, yes, the goal is to move everything that makes sense into the appropriate library. In the meantime, Gtk-OSX can provide add-on libraries like ige-mac-integration and patchfiles which are automatically applied by jhbuild to tarballs. Unfortunately, jhbuild has no provision for automatically patching VCS checkouts (which makes sense, because the risk is quite high that a patch won't apply cleanly against a
checkout unless the patch is in some dusty corner of the code).