You're right that FA signings have generally been very good and generally better than draftees. I don't credit Caserio for the FA signings however.
A little explanatory history that may be helpful for some.
There's a difference in Pro Personnel Director and Player Personnel Director. The director of pro personnel is responsible for evaluating players who no longer have college eligibility. He has 3-4 scouts who scout the NFL and finds players who are potential fits for his team in the future. The Pro Personnel director is also responsible for the weekly development of an ‘Advance Scouting Report’ with pertinent information regarding the upcoming opponent. Pro Personnel is also responsible for managing player workouts during the year. Caserio held that job for a very short time from 2004 - 2006 when he became director of player personnel. He left that & coached WRs in 2007 and in 2008 he was named the director of player personnel, a job he kept til yesterday. Caserio hasn't been in charge of Pro Personnel for 14 years.
Caserio has been the Pats Player Personnel Director for the last 12 years. His central job is evaluating college players and he must coordinate all the information on a player, including input from team scouts and coaches, recruiting services and other sources, to present to BB. He is also responsible for setting the draft board once players are identified. He's responsible for coordinating team travel, and player and team non-game public appearances, handling media coverage, as well as managing new player orientation.
Normally the GM handles contracts and cap strategy but for the Pats I think Caserio did the contracts and cap research. BB had final authority over draft picks while leaning on Caserio's reports & recommendations. BB is very hands on with regard to interviews.
Pro Personnel and Player Personnel Directors are normally equals within the executive staff hierarchy. Caserio's tenure gave him extra power and duties, no doubt.