Way back when I was in high school we were told that one of the few areas in which the Union was outmatched by the confederacy, at least early in the war, was in the quality of their Generals. I also understand that Lee was originally asked to be part of the union army but declined, feeling that although he opposed secession, he felt greater duty to his state of Virginia than the the Federal government. But what if he had taken Lincoln up on his offer? Would better leadership have allowed the union to achieve a relatively quick capture of Richmond, or would it have remained the long costly slog that it turned into.
On a more extreme note: Suppose, that despite the fact that it was totally against his character, that Lee became a union general under false pretenses, with the goal of sabotaging the war effort and bringing victory to the confederacy. How much damage could he have done before he was deemed incompetent and replaced?