Drawing with cairo-gl/glitz/etc was never really faster than drawing on the CPU. I think it's more likely that we'd use the image surface (backed by pixman) and texture uploads.
If we are hitting a path where we expose the immediate mode style drawing (a'la cairo) then we've probably already lost the performance race.
I hope for the CSS theme changes there is some sort of migration, or warning - was trying gnome-staging on Ubuntu and lots of things broke with the change.
Fingers crossed that GL will bring GL Enabled cairo at the same time.