I think the firms who were paying $1700 plus bonuses/bar study pay are going to $1900 with no bonus or bar study pay like McCarthys just for the optics of matching the weekly salary number. They may not all be doing that but I know some are.
It's entirely dependent on the firm. There are a couple structures that are quite common, and if I had to guess the 10/15/20% bonus structure is the most common on the street overall, but you really can't know without talking to associates at the firms. Some have a flat bonus for all associates. Some do flat bonuses for all that hit a certain billable target. Some do increasing bonuses based on billables, while others do increasing bonuses based on partner discretion.