Harassment is definitely a huge issue: there are tons of sleazy men who will harass wait staff secure in the knowledge that while their behavior is technically illegal most tipped employees are going to put up with it rather than miss on a big chunk of their income.

What about the reverse? Do those that deliver better service make more money than their under-performing peers?

Only if those providing better service are more attractive, more flirty (works only for the attractive, I suspect), touch their customers more, draw smiley faces on the bill, or participate in the various other things that do actually affect the tip.

But that's true in general. Attractive people have lots of advantages.

I wonder if anybody has ever done a study with a group of waiters where on alternate days they alter the quality of service they provide but nothing else. Are their tips constant?