Evil-J
The idea behind an REA is that ATC can tell us exactly when an aircraft is actually ready to taxi and can be airborne in a very short time. It is there to help ATC with sequencing as much as anything.
That is why only the tower can send an REA and they can specify the taxi time.
Sending a REA within the parameters stated mentioned in the previous reply is the best way to get an improvement.
The REA will work best if there is only one regulation as that gives the computer the best chance of finding an improvement. It is all done by the computer as opposed to a help desk call where we use our "skill and judgement" (!) to move flights forward or back.
Hope this helps
Prospero