I have not seen a statistical analysis, but you can use the database here
Draft Finder - Pro-Football-Reference.com
to look at second round picks, and get some idea of how they have done. CarAV (Career Approximate Value) is not going to be accurate for individuals, but it does give a good idea of how many very successful, and very unsuccessful, players there are in a particular draft.
My impression is that a 50% hit rate in the second round is very high and is almost undoubtedly as good as anyone's hit rate over a period of time.
You have to compare a general managers success with draft picks and with free agents with what the best of the GM's accomplish, not with some absolute standard of never missing.
Complaining about Belichick's drafting is like someone complaining that a baseball hitter with a batting average of .333 is not really a very good hitter. After all, he only gets a hit one out of three at bats. Surely a good hitter would be able to get a hit 4 out of 5 times.