How do you measure impairment with an habitual user? Do you have data before they started using frequently? Before they started using at all?
It's like saying breaking the second leg didn't significantly slow the racehorse.
I don’t really know the methodology, there was a link to it on one of these threads. I suppose if you were designing such a study, you could start with people who don’t use it, and have them do something that requires physical and mental coordination. Then you give them x amount of THC and test them again. Presumably they would be impaired. Then repeat that over time and see if they perform better on he drug as time goes on.