As someone who has watched Porcello and the Tigers a lot, I would say I would not pay him that much. It seems to almost be a running theme with Porcello. After another season of being a mediocre back of the rotation starter, fans would say he's still only 23, or 24, or 25. I would say at age 26 it is time to prove yourself. Porcello seemed really interested in playing in Boston. It may have been better to let this season play out, make a qualifying offer, and re-sign him. I doubt he would have left.
Porcello tends to get gassed after 100 pitches and fades quickly in games. Farrell will have to understand he is not a 120 pitch per game pitcher. He also doesn't have a great variation between his fastball and off speed pitches. The Tigers have generally had poor defensive teams but Doug Fister pitched well in front of that defense being a pitch to contact, ground ball pitcher. It seems like Porcello also has the tendency to unravel quickly. If it's in the 5th or 6th inning and he's given up a couple hits in a row, have to get him quick.