The fact that it is enhancing your performance and you are able to tolerate it is reason enough to not judge you for using it. If I could take a drug that made me significantly smarter, I would be willing to pay vast percentages of my income for it, like up to 50%. We must pathologize to justify enhancement. But there really should be no need. We all wish to improve ourselves. If responsible use of amphetamines betters you, that's enough to justify its use.