I remember reading in at least one study that there's no added benefit of doses above 1mg. I checked the "Brzezinski et al" article and it says:

  The dose response relationships in 11 of the studies 
  (see Refs. 17,18) support the existence of a plateau
  effect, with maximum effect generally being achieved
  at low doses (e.g. 0.3 mg) and maintained or 
  diminished at higher doses.