EXA is a new 2d acceleration architecture introduced in xorg 6.9/7.0. For nouveau, EXA is enabled by default; there is no support for XAA.
For more information: Click Here
EXA performance (especially with composite on) seems to improve with: (without it, an idle composite extension seems to negative effect on cpu usage)
Section "Device" in /etc/X11/xorg.conf:
Option "MigrationHeuristic" "smart"
Things like transparancy is still dirt slow though. Time will tell if this really helps.
Note: Xserver versions prior to 1.4.1 have a bug that can cause extreme EXA slowdowns.