I'm sorry, but I'm not sure any of that is true. Read this story.
https://www.baltimoresun.com/sports/orioles/bal-chris-davis-talks-about-suspension-and-his-add-20150131-story.html
He was diagnosed with ADHD in 2008 and began taking Adderall then. For whatever reason, he wasn't granted an exception in 2012 (when he was OK), in 2013 (when he was great) or in 2014 (when he was s***). He was busted for taking Adderall in 2014 and served a 25 game suspension. If you read between the lines, it sounds like he started taking Adderall again in 2014 - while knowing it was banned - as a crutch because he was battling injury and really struggling at the plate (and thus we looking for anything might help him).
He was given an exemption again from 2015 through the end of his career (I believe) so theoretically, he was on it throughout his unprecedented decline, which started at age 30 and he was completely useless at age 31.
Davis was also a beast and thus very well could have been on HGH - only to go 'off' it as soon as he got the big paycheck. I don't think he's a good example to use. If you google it, almost everything suggests steroids/HGH are significantly more performance enhancing than amphetamines.
If amphetamines enhanced performance that much - do you think MLB would grant exemptions and wouldn't like everyone in baseball be applying for them? I think it would be a MASSIVE debate every year if they were better than roids. Dr. Ned would be diagnosis the entire league with ADHD so they could get an exemption.