Effect on performance
The effects of alcohol on performance differ from individual to individual. A "couple" (depending on what I'm having) of drinks absolutely enhances my experience. I may even have a couple more if my date is drinking as well.
Note* Many SP's will not have anything at all if it's not from an unopened bottle. That's perfectly understandable and that's why I always keep a couple of fresh sodas, juices, waters, and a bottle of wine available.
Note** Some agencies prohibit their ladies from drinking at all on an appointment.
Once, many years ago, I drank to excess at a reception when I had a 3 hour committment for later that evening. When the lady showed up, I paid her, had one fast, sloppy SOG, and then then proceeded to pass-out cold. To her credit, she stayed the entire time, watched TV, and then woke me up to tell me she was leaving. I was fortunate that she was an honest young lady becuase she could have cleaned out the room (or my wallet, at the very least) and left at her leisure.