"Higher doses will lead to headaches and, paradoxically, tiredness and drowsiness — not stimulation."

False. You're right in that it won't help them very much -- specifically because taking overdoses of amphetamines tends to lead to things like insomnia, tachycardia/dehydration, and mania/psychosis.

The effects I described were experienced by me during the calibration period, where I took higher dose than me and my therapist eventually settled on. Perhaps it might work differently for some people, but this is what I heard from other people who suffer from ADHD and tried to take higher doses.