Please read the posting guidelines, we're not here to tell you what to say to a doctor to acquire drugs. Street methamphetamine is going to be less pure (obviously) than Desoxyn, which is pure
methamphetamine, USP. Exact purity of street methamphetamine is going to vary from source to source and from dealer to dealer...so we can't give you a straight answer on that.
Yes, methamphetamine (Desoxyn) is more potent than dextroamphetamine (Dexedrine). Methamphetamine is longer lasting and has more action on serotonin release (making it more euphoric) than dextroamphetamine or racemic amphetamine/mixed amphetamine salts (Adderall).
Desoxyn is considered the last resort for ADD/ADHD treatment and many doctors will not prescribe it because of the stigma associated with methamphetamine. In addition, many pharmacies do not stock Desoxyn for obvious reasons.