Unfortunately you can pretty much nanny-pick a study that supports your view for just about any purported topic and view relating to drug use.
Secondly, that study is flawed because it fails to acknowledge what is known as the 30%-30%-30% jump. The difference in intelligence between a retard and a normal person is the same difference of intelligence between a normal person and a genius. Different boosts account for different corrections in deficits depending on the IQ range that is being operated on. This is a known quirk of intellect. Being why Einstein sent a letter to his son telling him that intelligence was not the reason he was successful, "working hard" was, and urging him to do the same.
Amphetamines are performance enhancing drugs. They give the same "cracked out" superman effect to anyone who takes them. They are amazing. And we should not discount them by giving credence to the bullshit that is ADD and the epidemic which is giving dumb kids speed so they can live up to their parents' standards.
ADD is overdiagnosed, and adderall/speed is overprescribed in doses that hit high enough that their grandma can feel it. It's fucked up to be giving hard drugs to kids just because the kids are dumb. Let them be dumb. Let's not make them drug addicts at the cost of their mental freedom.
Just google for it, the stories on forums and other places. There are millions of kids who grew up and regret the drug issues they have now. The dependencies. Because they've been on speed their whole lives, because of a giddy parent and a giddy psychiatrist long long ago. The stories show that treating "ADD" with speed in children, is a really shitty thing to do.