There can never be enough research (or so it seems):

Yet another paper has been published of a study whose findings confirm what has been known for decades: "High doses of methadone greater than 100 mg daily may provide a better outcome for illicit opiate use among some patients who would not respond to moderate doses." To reach this conclusion seven authors collaborated in a study involving a total of 59 (!) patients in treatment at the Atlanta (Ga.) VA Hospital.

Source: J Addictive Dis 28:399-405, 2009