When doing work on CT_UnitFrames earlier this year, I found one of the biggest performance improvements came from taking old circa Burning Crusade code like this:
Lua Code:
RegisterEvent("UNIT_HEALTH")
and replacing it with a function introduced in Mists of Pandaria:
Lua Code:
RegisterUnitEvent("UNIT_HEALTH", "target")
I even noticed a difference alone in front of a combat dummy, when measuring CPU usage in a controlled setting. I never extended those measurements to a true raid setting, but the impact would probably be greatly magnified by group size and in larger pulls.