I have experienced this increase in alertness with both a moderate amount of alcohol and a friend on the line with skype. I think the benefit has something to do with my tendency to get distracted. If I am alone with my thoughts and sober I will be thinking/overthinking all sorts of things while in game. If I've had a few or am chatting merrily about his or that with a friend, I simply choose a direction, and all my subsequent movements are efficient reactions to in-game encounters.
As for skill dropoff, unless I've had too MUCH to drink, my worst moments are when I first get on and havent acclimated to the game yet. Often Ill do so badly Ill have to start a squad and focus on NOT DYING for a time. Which can still be quite helpful to the team, of course.