If Blizzard has to do it aswell: maybe some hint can be found in FrameXML/BuffFrame.lua, who knows.
Oh...hmm
http://www.wowwiki.com/API_GameTooltip_SetUnitAura
http://wowprogramming.com/docs/widge...ip/SetUnitAura
So the tooltip is actually done server side.
But maybe there is a new value in
http://wowprogramming.com/docs/api/UnitAura that has the numbers.