I've experienced a number of players who claim that they play better while drunk/high. This may or may not be true; since they've always been drunk or high while I've encountered them in-game, I never have a "sober version" against which to compare their performance, so I can only take them at their word. But of these players who supposedly play better while drunk/high I've yet to find one that plays WELL while drunk/high.

I think it's less likely that they honestly believe that intoxication improves their performance, and much more likely that they just like being intoxicated in general and use this as an excuse when folks ask why they're slurring into their microphones.

If they could get away with an excuse like this outside the game, I'm pretty sure they'd also claim that they sing better when drunk/high, operate heavy construction equipment better while drunk/high, perform neurosurgery while drunk/high...