Let's travel back to 1965 when these drugs were legal. A nurse I know mentioned how Adderall was freely available at the nurses station, and after a period of experimentation (I believe she made mention of working a 60 hour week), most everyone dialed it the fuck down, and its use was mostly relegated to having a case of the Mondays, with a few burnouts here and there. This was also when three martini lunches were in vogue. Can you risk not having a few drinks with your business partners? Would you really call that performance enhancing?
Fact is our drug war has been the response to already going down this road before, and in case the evidence from Portugal isn't clear, most people tend to reduce there drug use across the board when they are legal.
The other side to performance enhancing drugs is that they tend to increase your ability to do physical labor, but otherwise they make you sloppy. 100 billable hours isn't much good if only 25 of them are useful, and you don't see meth heads rising to the top of industry or otherwise courted for employment.
The biggest factor in their use now is that they are illegal, and only a select few can pull the right strings to get them, which gives them a temporary advantage.
But if they are freely available? Most people still won't touch them, and are capable enough that even a slight increase in other's performance (at best, they will land you 5-10 more points on a standardized test) is indistinguishable from natural variation. Especial in the case of speed, there has been enough scaremongering (speed kills) regarding that is laughable. Drugs are a fun tangent, but eventually real, clear-headed work needs to be done.