The elixir does stack with RAF and whatever else you've got. With a single elixir on the character able to grant levels (you don't both need one) we went 68-80 in 36 minutes queuing into dungeons. He was tanking; I was dps. Ironically there was a several minute wait between dungeons for a healer and we still achieved the time mentioned above.
Most people use Stonecore boost runs to completely skip Cataclysm leveling (80-85). An RAF pair could comfortably go 68-85 in the one hour window of the elixir with very minor planning.