It has definitely been the plan for a long time to get all drivers onto EXA. XAA is only receiving bug fixes in the server. On the other hand, UXA is used by the intel driver to play nice with GEM and KMS. It started as a straight clone of EXA, though. I don't know if the UXA features can be ported back to EXA in the server. You'd probably have to ask keithp or anholt.