Just a couple of points regarding Porcello:
–Boston actually acquired him via trade, not free agency. They got him for a package including Yoenis Cespedes. The big money came just as the season was getting underway, a contract extension worth 4 yrs/$82 million.
–My recollection is that the trade was well received (on Boston’s behalf) by a lot of commentators, though not because of Porcello’s advanced stats. As RickJay points out, Porcello was very young and had had one pretty good year, so there was reason to think that he might be pretty good. Also, the consensus was that Boston was right to get rid of Cespedes, who was a time bomb ready to go off or something. Plus he had a low OBP. And in general I think there was a perception that the Red Sox are a smart organization that usually makes good moves, and so they got the benefit of the doubt.
–The wisdom of the contract extension was more debatable (again, going from memory here), but there were certainly people who defended it on the grounds cited above by several posters: Porcello had definite upside, Boston has a Trump-sized payroll and can afford a bad contract in a way that most teams can’t.
Anyway, based on what we know about advanced stats, the Porcello trade and extension probably didn’t make an awful lot of sense. Should be mentioned though that many teams (all?) have their own proprietary analytics, and Boston’s may have seen something in Porcello’s numbers and profile that the ones available to us, the fans, don’t see.
[on review, I see some of this was covered by DaleSams.]