This is exactly what several electric "bicycles" do now. They have a road mode which conforms to the federal 750W/20MPH limit, and an offroad mode that allows something like several kilowatts and perhaps 40MPH.
Or they could be Volkswagened and say that it puts out 1000w when it really does 2000 :)
That would kind of be anti-Volkswagening, in that Volkswagen cars had more power than they would have, had they not cheated on emissions standards.