Well... the ideal temperature for decarboxiling the weed is 105C (221F), far below the temperatures required to vaporize stuff from the weed. Probably Master Wu used higher temperatures because the heating of the weed in a pan or even in the oven is not uniform, and so for ensure that even the more inner parts of the weed would reach this ideal temperature he would need to heat more the outside parts of it. Also, weed always contains moisture, which takes a large amount of heat to evaporate, so it would also increase the need of heating it more.
Anyway, the best method for decarboxilating the weed is just microwaving it for some minutes (2-3). The microwave doesnt heat it far above the waters boiling point (212F) and so it doesnt boil off any good stuff from the weed. Only remember to microwave the weed before grinding it, or it can (due unknown reasons for me) catch fire.