People at grad school with my husband used to work as wait staff, and do really well - better than I was doing as a senior lab tech - but that was in a city with good restaurants and some population with good bank accounts.
On the other hand, I've frequented many kinds of restaurants, from apparent gourmet paradises to many diners and "holes in the wall", which are my favorites when they're good. When one's whole meal costs under six dollare (a trick doable with biscuits and gravy and coffee or eggs and coffee at some places), leaving 20% is, ah, dinky.
Especially back in what I called north north - some restaurants had relatively few customers and had trouble keeping staff - a pretty good mexican restaurant on highway 101, which is a major highway - with a mother and two sons handling everything, one son the main cook, if he was around, the mother the second cook and hostess, the surly (what? quite young teen) bringing menus and water and bussing. I always way over tipped re percentages, but it was tough for them to keep open.
On another subject, re chefs making good money. Some do, some don't, from my reading. I've read a few books by chefs/restaurant owners and remember talk of resentment between the line cooks and the wait staff re money earned.
No links though.