You said yourself in another post that you cannot assume that the productivity gains seen are evenly distributed. So I would propose that there is definitely an informational advantage for the company that currently employs Bob, because they know how much value Bob produces over his cost. How is another company supposed to gain this information, short of hiring Bob and finding out themselves? In which case, they rationally can really only be willing to pay Bob at some sort of expected average productivity.

The only information leak is how much Bob is currently paid. So in some ways it's actually to a company's advantage to basically average out the wages across a level based on the level's entire productivity, because it hides the truly productive from other companies. And this is exactly what we see with modern pay grades and scales.