I can hardly add anything after Raedwulf's good post.
If you are relaxed and feel confident, the optimal performance will be at an alcohol level close to nil or nil (I don't want to go into why 'close to' here, but that means at most half a pint).
With more than that your motor performance goes down, at first slowly, then rapidly. For those who can relax better with alcohol, they are getting better (due to confidence etc.) with an increasing amount of alcohol until the deterioration of motor performance takes away the benefit from confidence. This optimal point for those who need the relaxing effect is a very personal point and one pint too much can be davastating to your performance.
As for well known musicians who drank a lot, that only shows how good they were (as musicians) that even under impeding circumstances they still could perform fine. But they'd have been even better without the booze, at least without it in these amounts.