FWIW, here's what we estimate NBA replacement level to be based on the performance of players on two-way contacts. About -2.75 points per 100 possessions, of which -1.85 comes on offense and -0.90 on defense, although the offense/defense split varies by position. 
We don't buy that there should be different replacement levels across different positions. Instead, stats should be designed such that the league average at each position is roughly equal. Many advanced stats are biased toward big men, especially traditional post-up bigs.
We do find that PGs are generally more valuable than shooting guards, for the same reason that shortstops are generally more valuable than second basemen. The best PGs can generally do all the things that SGs can do, plus run the offense and generate additional value that way.
